Semiconductor device, display device, and electronic appliance

ABSTRACT

In case the size of the transistor is enlarged, power consumption of the transistor is increased. Thus, the present invention provides a display device capable of preventing a current from flowing to a display element in signal writing operation without varying potentials of power source lines for supplying a current to the display element per row. In setting a gate-source voltage of a transistor by applying a predetermined current to the transistor, a potential of a gate terminal of the transistor is adjusted so as to prevent a current from flowing to a load connected to a source terminal of the transistor. Therefore, a potential of a wire connected to the gate terminal of the transistor is differentiated from a potential of a wire connected to a drain terminal of the transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/716,840, filed Mar. 3, 2010, now allowed, which is a continuation ofU.S. application Ser. No. 11/425,242, filed Jun. 20, 2006, now U.S. Pat.No. 7,679,585, which claims the benefit of a foreign priorityapplication filed in Japan as Serial No. 2005-191145 on Jun. 30, 2005,all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having afunction to control a current to be supplied to a load by a transistor,and a display device including a pixel formed of a current drivendisplay element in which luminance is changed in accordance with asignal, and a signal line driver circuit and a scan line driver circuitfor driving the pixel. Further, the present invention relates to adriving method of the display device. Furthermore, the present inventionrelates to an electronic appliance having the display device as adisplay portion.

2. Description of the Related Art

A liquid crystal display (LCD) which is a display device formed of adisplay element such as a liquid crystal is widely used. On the otherhand, in recent years, a so-called self-light emitting display devicehaving a pixel which is formed of a display element such as a lightemitting diode (LED) has been attracting attention. As a display elementused for such a self-light emitting display device, an organic lightemitting diode (OLED) (also referred to as an organic EL element,electroluminescence (EL) element, or the like) has been attractingattentions, and they have been used for an EL display and the like. Adisplay element such as an OLED is self-light emitting, therefore, ithas advantages such as higher visibility of pixels, no backlight, andhigher response compared to a liquid crystal display. Note that theluminance of a display element is generally controlled by a currentvalue flowing through it.

As a driving method for expressing a gray scale of such a displaydevice, there are a digital gray scale method and an analog gray scalemethod. By the digital gray scale method, a display element is turnedon/off by being controlled in a digital manner to express a gray scale.In the case of the digital gray-scale method, the uniformity inluminance of each pixel is excellent; however, only two gray scalelevels can be expressed if nothing is done since there are only twostates, that is, light emission and non-light emission. Therefore,another method is used in combination to realize a multi-level grayscale. There is an area gray-scale method to express a gray scale byselecting the weighted light emission areas of pixels and a timegray-scale method to express a gray scale by selecting the weightedlight emission time. Further, in the case of the digital gray-scalemethod, a time gray-scale method which is appropriate for achieving highdefinition is often employed. On the other hand, as the analoggray-scale method, there are a method of controlling the light emissionintensity of a display element in an analog manner, and a method ofcontrolling the light emission time of a display element in an analogmanner. As the analog gray scale method, often employed is a method tocontrol light emission intensity of a display element in an analogmanner. As the method to control light emission intensity in an analogmanner, often employed is a current input current driving method whichis hardly affected by variations in characteristics of a thin filmtransistor (hereinafter also referred to as a TFT) of each pixel.

A current input current driven pixel including a unipolar transistor,that is, a transistor having one of a p-channel polarity or an n-channelpolarity is disclosed in Patent Document 1 and Non-Patent Document 1.

-   [Patent Document 1] Japanese Patent Laid-Open No. 2004-021219-   [Non-Patent Document 1] SID 04 DIGEST p. 1516-p. 1519

SUMMARY OF THE INVENTION

According to Patent Document 1 and Non-Patent Document 1, potentials ofpower source lines for supplying a current to a display element arevaried per row; thereby preventing a current from flowing to a displayelement when signals are written to pixels. If a current flows to adisplay element in signal writing operation, the signals cannot becorrectly written to pixels. As a result, a display defect occurs.

Meanwhile, a large amount of current is required to be supplied to alight emitting element from the power source lines. Therefore, a switchcapable of controlling a large amount of current is required to bedisposed to supply the large amount of current while varying potentialsof the power source lines per row. Because of this, it is a problem thatthe size of a transistor in a circuit is necessarily enlarged. If thesize of the transistor is enlarged, power consumption of the transistoris increased.

Further, in the case of the prior structure as described in Non-PatentDocument 1 and Patent Document 1, Vds=Vgs is satisfied in a transistorfor driving a display element in signal writing operation. On the otherhand, in light emission operation, Vds>Vgs is satisfied. Therefore, whenconstant current characteristics (flatness of current) in a saturationregion are deteriorated, a current value significantly varies between inthe signal writing operation and in the light emitting operation.

Thus, the present invention provides a display device capable ofpreventing a current from flowing to a display element in signal writingoperation without varying potentials of power source lines for supplyinga current to the display element per row.

In the present invention, in setting a gate-source voltage of atransistor by applying a predetermined current to the transistor, apotential of a gate terminal of the transistor is adjusted so as toprevent a current from flowing to a load connected to a source terminalof the transistor. Therefore, a potential of a wire connected to thegate terminal of the transistor is differentiated from a potential of awire connected to a drain terminal of the transistor.

That is to say, the potential of the gate terminal of the transistor isset to be higher or lower than the potential of the drain terminal ofthe transistor, thereby adjusting a potential of a source terminal ofthe transistor, and preventing a current from flowing to the load.

Hereinafter, specific structures are described.

A semiconductor device of the present invention includes a transistor, afirst switch, a second switch, a capacitor, a first wire, a second wire,a third wire, and a load. A first terminal of the transistor isconnected to the first wire through the first switch, a second terminalof the transistor is connected to the second wire, and a gate terminalof the transistor is connected to the third wire through the secondswitch. The capacitor is connected between the gate terminal and thefirst terminal of the transistor. The load is connected to the firstterminal of the transistor.

Further, in the semiconductor device of the present invention having theabove structure, a predetermined potential is inputted to the secondwire and the third wire.

A semiconductor device of the present invention includes a transistor, afirst switch, a second switch, a capacitor, a first wire, a second wire,a third wire, and a load. A first terminal of the transistor isconnected to the first wire through the first switch, a second terminalof the transistor is connected to the second wire, and a gate terminalof the transistor is connected to the third wire through the secondswitch. The capacitor is connected between the gate terminal and thefirst terminal of the transistor. The load is connected to the firstterminal of the transistor. A potential of the third wire is lower thanthat of the second wire.

A semiconductor device of the present invention includes a transistor, afirst switch, a second switch, a capacitor, a first wire, a second wire,a third wire, and a load. A first terminal of the transistor isconnected to the first wire through the first switch, a second terminalof the transistor is connected to the second wire, and a gate terminalof the transistor is connected to the third wire through the secondswitch. The capacitor is connected between the gate terminal and thefirst terminal of the transistor. The load is connected to the firstterminal of the transistor. A predetermined potential is inputted to thesecond wire and the third wire. When the first switch and the secondswitch are turned on and a current flows to the first wire, a currentflows to the transistor and does not flow to the load. Meanwhile, whenthe first switch and the second switch are turned off, a current flowsto the transistor and the load.

A semiconductor device of the present invention includes a firsttransistor, a second transistor, a third transistor, a capacitor, afirst wire, a second wire, a third wire, a fourth wire, and a load. Afirst terminal, a second terminal, and a gate terminal of the firsttransistor are connected to a second terminal of the second transistor,the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The load is connected to thefirst terminal of the first transistor.

Further, in the semiconductor device of the present invention having theabove structure, a predetermined potential is inputted to the third wireand the fourth wire.

A semiconductor device of the present invention includes a firsttransistor, a second transistor, a third transistor, a capacitor, afirst wire, a second wire, a third wire, a fourth wire, and a load. Afirst terminal, a second terminal, and a gate terminal of the firsttransistor are connected to a second terminal of the second transistor,the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The load is connected to thefirst terminal of the first transistor. A potential of the fourth wireis lower than that of the third wire.

A semiconductor device of the present invention includes a firsttransistor, a second transistor, a third transistor, a capacitor, afirst wire, a second wire, a third wire, a fourth wire, and a load. Afirst terminal, a second terminal, and a gate terminal of the firsttransistor are connected to a second terminal of the second transistor,the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The load is connected to thefirst terminal of the first transistor. When the second transistor andthe third transistor are turned on by a signal inputted to the firstwire and a current flows to the first wire, a current flows to the firsttransistor and does not flow to the load. Meanwhile, when the secondtransistor and the third transistor are turned off, a current flows tothe first transistor and the load.

A display device of the present invention includes a first transistor, asecond transistor, a third transistor, a capacitor, a first wire, asecond wire, a third wire, a fourth wire, and a light emitting elementhaving a light emitting layer between a pixel electrode and an opposedelectrode. A first terminal, a second terminal, and a gate terminal ofthe first transistor are connected to a second terminal of the secondtransistor, the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor.

Further, in the display device of the present invention having the abovestructure, a predetermined potential is inputted to the third wire andthe fourth wire.

A display device of the present invention includes a first transistor, asecond transistor, a third transistor, a capacitor, a first wire, asecond wire, a third wire, a fourth wire, and a light emitting elementhaving a light emitting layer between a pixel electrode and an opposedelectrode. A first terminal, a second terminal, and a gate terminal ofthe first transistor are connected to a second terminal of the secondtransistor, the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor. A potential of the fourth wire is lower than that of thethird wire.

A display device of the present invention includes a first transistor, asecond transistor, a third transistor, a capacitor, a first wire, asecond wire, a third wire, a fourth wire, and a light emitting elementhaving a light emitting layer between a pixel electrode and an opposedelectrode. A first terminal, a second terminal, and a gate terminal ofthe first transistor are connected to a second terminal of the secondtransistor, the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor. When the second transistor and the third transistor areturned on by a signal inputted to the first wire and a current flows tothe first wire, a current flows to the first transistor and does notflow to the light emitting element. Meanwhile, when the secondtransistor and the third transistor are turned off, a current flows tothe first transistor and the light emitting element.

A display device of the present invention includes a first transistor, asecond transistor, a third transistor, a capacitor, a first wire, asecond wire, a third wire, a fourth wire, and a light emitting elementhaving a light emitting layer between a pixel electrode and an opposedelectrode. A first terminal, a second terminal, and a gate terminal ofthe first transistor are connected to a second terminal of the secondtransistor, the third wire, and a first terminal of the third transistorrespectively. A gate terminal and a first terminal of the secondtransistor are connected to the first wire and the second wirerespectively. A gate terminal and a second terminal of the thirdtransistor are connected to the first wire and the fourth wirerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor. A potential of the fourth wire is equal to that of theopposed electrode.

A display device of the present invention includes a scan line drivercircuit, a signal line driver circuit, and a pixel portion. The pixelportion includes a plurality of scan lines extended from the scan linedriver circuit, a plurality of signal lines extended from the signalline driver circuit, and a plurality of pixels arranged in matrixcorresponding to the scan lines and the signal lines. Each of the pixelsincludes a first transistor, a second transistor, a third transistor, acapacitor, a scan line, a signal line, a power source line, a bias line,and a light emitting element having a light emitting layer between apixel electrode and an opposed electrode. A first terminal, a secondterminal, and a gate terminal of the first transistor are connected to asecond terminal of the second transistor, the power source line, and afirst terminal of the third transistor respectively. A gate terminal anda first terminal of the second transistor are connected to the scan lineand the signal line respectively. A gate terminal and a second terminalof the third transistor are connected to the scan line and the bias linerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor.

A display device of the present invention includes a scan line drivercircuit, a signal line driver circuit, and a pixel portion. The pixelportion includes a plurality of scan lines extended from the scan linedriver circuit, a plurality of signal lines extended from the signalline driver circuit, and a plurality of pixels arranged in matrixcorresponding to the scan lines and the signal lines. Each of the pixelsincludes a first transistor, a second transistor, a third transistor, acapacitor, a scan line, a signal line, a power source line, a bias line,and a light emitting element having a light emitting layer between apixel electrode and an opposed electrode. A first terminal, a secondterminal, and a gate terminal of the first transistor are connected to asecond terminal of the second transistor, the power source line, and afirst terminal of the third transistor respectively. A gate terminal anda first terminal of the second transistor are connected to the scan lineand the signal line respectively. A gate terminal and a second terminalof the third transistor are connected to the scan line and the bias linerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor. A potential of the bias line is lower than that of the powersource line.

A display device of the present invention includes a scan line drivercircuit, a signal line driver circuit, and a pixel portion. The pixelportion includes a plurality of scan lines extended from the scan linedriver circuit, a plurality of signal lines extended from the signalline driver circuit, and a plurality of pixels arranged in matrixcorresponding to the scan lines and the signal lines. Each of the pixelsincludes a first transistor, a second transistor, a third transistor, acapacitor, a scan line, a signal line, a power source line, a bias line,and a light emitting element having a light emitting layer between apixel electrode and an opposed electrode. A first terminal, a secondterminal, and a gate terminal of the first transistor are connected to asecond terminal of the second transistor, the power source line, and afirst terminal of the third transistor respectively. A gate terminal anda first terminal of the second transistor are connected to the scan lineand the signal line respectively. A gate terminal and a second terminalof the third transistor are connected to the scan line and the bias linerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. When the second transistorand the third transistor are turned on by a signal inputted to the scanline and a signal current flows to the signal line, a current flows tothe first transistor and does not flow to the light emitting element.Meanwhile, when the second transistor and the third transistor areturned off, a current flows to the first transistor and the lightemitting element.

A display device of the present invention includes a scan line drivercircuit, a signal line driver circuit, and a pixel portion. The pixelportion includes a plurality of scan lines extended from the scan linedriver circuit, a plurality of signal lines extended from the signalline driver circuit, and a plurality of pixels arranged in matrixcorresponding to the scan lines and the signal lines. Each of the pixelsincludes a first transistor, a second transistor, a third transistor, acapacitor, a scan line, a signal line, a power source line, a bias line,and a light emitting element having a light emitting layer between apixel electrode and an opposed electrode. A first terminal, a secondterminal, and a gate terminal of the first transistor are connected to asecond terminal of the second transistor, the power source line, and afirst terminal of the third transistor respectively. A gate terminal anda first terminal of the second transistor are connected to the scan lineand the signal line respectively. A gate terminal and a second terminalof the third transistor are connected to the scan line and the bias linerespectively. The capacitor is connected between the gate terminal andthe first terminal of the first transistor. The pixel electrode of thelight emitting element is connected to the first terminal of the firsttransistor. A potential of the bias line is equal to that of the opposedelectrode.

Further, in the display device of the present invention having the abovestructure, the first, second, and third transistors are n-channeltransistors.

Further, in the display device of the present invention having the abovestructure, an amorphous semiconductor film is used for a semiconductorlayer of the n-channel transistor.

A switch used in the invention may be any switch such as an electricalswitch or a mechanical switch. That is, it may be anything as far as itcan control a current flow and is not limited to a particular type. Itmay be a transistor, a diode (PN diode, PIN diode, Schottky diode,diode-connected transistor, or the like), or a logic circuit configuredwith them. Therefore, in the case of using a transistor as a switch,polarity (conductivity) thereof is not particularly limited because itoperates just as a switch. However, when an off current is preferred tobe small, a transistor of polarity with a small off current ispreferably used. For example, a transistor which has an LDD region or amulti-gate structure has a small off current. Further, it is desirablethat an n-channel transistor be employed when a potential of a sourceterminal of the transistor operating as a switch is closer to a lowpotential side power source (Vss, GND, 0 V or the like), and a p-channeltransistor be employed when a potential of the source terminal is closerto the high potential side power source (Vdd or the like). This helpsthe switch operate efficiently since the absolute value of thegate-source voltage of the transistor can be increased. It is to benoted that a CMOS switch can also be applied by using both n-channel andp-channel transistors. With a CMOS switch, operation can beappropriately performed even when the situation changes such that avoltage outputted through a switch (that is, an input voltage to theswitch) is higher or lower than an output voltage.

In the invention, “being connected” means “being electrically connected”and “being directly connected”. Therefore, in the structure disclosed inthe invention, another element which enables an electrical connection(for example, switch, transistor, capacitor, inductor, resistor, diode,or the like) may be provided in addition to the predeterminedconnection. Alternatively, a direct connection may be made withoutinterposing another element. It is to be noted that when elements areconnected without interposing another element which enables anelectrical connection and connected not electrically but directly, it isreferred to as “being directly connected”. It is to be noted that “beingelectrically connected” means “being electrically connected” and “beingdirectly connected”.

Note that various modes can be applied to a display element. Forexample, a display medium in which contrast is changed by anelectromagnetic effect can be used, such as an EL element (organic ELelement, inorganic EL element, EL element containing organic materialand inorganic material), an electron discharging element, a liquidcrystal element, an electron ink, a light diffraction element, adischarging element, a digital micromirror device (DMD), a piezoelectricelement, or a carbon nanotube. It is to be noted that an EL panel typedisplay device using an EL element includes an EL display; a displaydevice using an electron discharging element includes a field emissiondisplay (FED), an SED type flat panel display (Surface-conductionElectron-emitter Display), and the like; a liquid crystal panel typedisplay device using a liquid crystal element includes a liquid crystaldisplay; a digital paper type display device using an electron inkincludes electronic paper; a display device using a light diffractionelement includes a grating light valve (GLV) type display; a PDP (PlasmaDisplay Panel) type display using a discharging element includes aplasma display; a DMD panel type display device using a micromirrorelement includes a digital light processing (DLP) type display device; adisplay device using a piezoelectric element includes a piezoelectricceramic display; a display device using a carbon nanotube includes anano emissive display (NED) and the like.

It is to be noted that transistors of various modes can be applied to atransistor of the invention. Therefore, kinds of transistors applicableto the invention are not limited. Accordingly, the following transistorsare applicable to the invention: a thin film transistor (TFT) using anon-single crystalline semiconductor film typified by amorphous siliconand polycrystalline silicon; a MOS transistor which is formed using asemiconductor substrate or an SOI substrate; a junction transistor; abipolar transistor; a transistor using a compound semiconductor such asZnO or a-InGaZnO; a transistor using an organic semiconductor or acarbon nanotube; and other transistors. It is to be noted that anon-single crystalline semiconductor film may contain hydrogen orhalogen. A substrate over which a transistor is provided is not limitedto a particular type and various kinds of substrates can be used.Therefore, a transistor can be provided over, for example, a singlecrystalline substrate, an SOI substrate, a glass substrate, a quartzsubstrate, a plastic substrate, a paper substrate, a cellophanesubstrate, a stone substrate, a stainless steel substrate, a substratehaving stainless steel foil, or the like. Further, a transistor formedover a certain substrate may be transferred to another substrate.

It is to be noted that a transistor can have structures of various modesand is not limited to a specific structure. For example, a multi-gatestructure where the number of gates is two or more may be employed aswell. With a multi-gate structure, an off current can be reduced andreliability can be improved by improving the pressure resistance of atransistor, and further flat characteristics can be obtained since adrain-source current does not change so much even when a drain-sourcevoltage changes in the operation in a saturation region. Further, gateelectrodes may be provided over and under a channel. Accordingly, achannel region increases, thereby a current value is increased or asubthreshold coefficient can be improved since a depletion layer iseasily formed. Further, a gate electrode may be provided over or underthe channel. Either a forward staggered structure or an inverselystaggered structure may be employed. A channel region may be dividedinto a plurality of regions, or connected in parallel or in series.Further, a source electrode or a drain electrode may overlap a channel(or a part of it), thereby preventing a charge from being accumulated ina part of the channel and unstable operation. Further, an LDD region maybe provided. By providing an LDD region, an off current can be reducedand reliability can be improved by improving the pressure resistance ofa transistor, and further flat characteristics can be obtained since adrain-source current does not change so much even when a drain-sourcevoltage changes in the operation in a saturation region.

It is to be noted that various types of transistors can be used as atransistor of the invention and formed over various substrates.Therefore, all of the circuits may be formed over a glass substrate, aplastic substrate, a single crystalline substrate, an SOI substrate, orany substrate. When all the circuits are formed over a substrate, costcan be reduced by reducing the number of components and reliability canbe improved by reducing the number of connections with the components.Alternatively, a part of a circuit may be formed over a certainsubstrate and another part of the circuit may be formed over anothersubstrate. That is, not all of the circuits are required to be formedover the same substrate. For example, a part of a circuit may be formedover a glass substrate using a transistor and another part of thecircuit may be formed over a single crystalline substrate into an ICchip which may be provided over the glass substrate by COG (Chip OnGlass). Alternatively, the IC chip may be connected to a glass substrateusing TAB (Tape Auto Bonding) or a printed substrate. In this manner,when parts of a circuit are formed over the same substrate, cost can bereduced by reducing the number of components and reliability can beimproved by reducing the number of connections with the components.Further, a portion with a high driving voltage or a high drivingfrequency which consumes more power is not formed over the samesubstrate, thereby an increase in power consumption can be prevented.

It is to be noted in the invention that one pixel corresponds to oneelement which can control brightness. Therefore, for example, one pixelexpresses one color element by which brightness is expressed.Accordingly, in the case of a color display device formed of colorelements of R (red), G (green), and B (blue), the smallest unit of animage is formed of three pixels of an R pixel, a G pixel, and a B pixel.It is to be noted that a color element is not limited to be formed ofthree colors and may be formed of more than three colors such as RGBW (Wis white) or RGB to which yellow, cyan, and magenta are added. Further,as another example, in the case of controlling the brightness of onecolor element by using a plurality of regions, one of the plurality ofregions corresponds to one pixel. Therefore, for example, in the case ofperforming an area gray scale display, a plurality of regions forcontrolling the brightness are provided for one color element, whichexpress a gray scale as a whole. One of the regions for controlling thebrightness corresponds to one pixel. Therefore, in this case, one colorelement is formed of a plurality of pixels. Moreover, in this case,regions which contribute to display may differ in size depending on thepixel. In the plurality of regions for controlling the brightnessprovided for one color element, that is, a plurality of pixels whichconstitute one color element, the viewing angle may be expanded bysupplying each pixel with a slightly different signal.

It is to be noted in the invention that pixels may be arranged inmatrix. Here, the case where pixels are arranged in matrix correspondsto a case where pixels are arranged in a grid pattern where longitudinalstripes and lateral stripes cross each other or to a case where dots ofthree color elements are arranged in what is called a delta pattern orin a Bayer pattern when a full color display is performed using thethree color elements (for example, RGB). It is to be noted that a colorelement is not limited to three colors and the number of colors may bemore than three. The size of a light emission area may be differentdepending on the dot of the color element.

It is to be noted that a transistor is an element having at least threeterminals including a gate, a drain, and a source. A channel region isprovided between a drain region and a source region. Here, it isdifficult to determine the source region or the drain region since theydepend on the structure, operating condition, and the like of thetransistor. Therefore, in the invention, a region functioning as asource or a drain may not be referred to as a source or a drain. In thiscase, for example, each of the region functioning as a source and theregion functioning as a drain may be referred to as a first terminal ora second terminal.

It is to be noted that a gate includes a gate electrode and a gate wire(also referred to as gate line, gate signal line, or the like) or a partof them. A gate electrode corresponds to a conductive film of a partoverlapping a semiconductor forming a channel region, an LDD (LightlyDoped Drain) region, and the like with a gate insulating film interposedtherebetween. A gate wire corresponds to a wire for connecting gateelectrodes of pixels and connecting a gate electrode and another wire.

However, there is a portion which functions as a gate electrode and alsoas a gate wire. Such a region may be referred to as a gate electrode ora gate wire. That is, there is a region which cannot be specificallydetermined as a gate electrode or a gate wire. For example, when thereis a channel region overlapping a gate wire which is extended, theregion functions as a gate wire and also as a gate electrode. Therefore,such a region may be referred to as a gate electrode or a gate wire.

Further, a region which is formed of the same material as a gateelectrode and connected to the gate electrode may be referred to as agate electrode as well. Similarly, a region which is formed of the samematerial as a gate wire and connected to a gate wire may be referred toas a gate wire. In a strict sense, such regions do not overlap a channelregion or do not have functions to connect to another gate electrode insome cases. However, there is a region which is formed of the samematerial as a gate electrode or a gate wire and connected to the gateelectrode or the gate wire due to a manufacturing margin and the like.Therefore, such a region may be referred to as a gate electrode or agate wire.

For example, in a multi-gate transistor, gate electrodes of onetransistor and another transistor are often connected through aconductive film formed of the same material as the gate electrodes. Sucha region may be referred to as a gate wire since it is a region forconnecting the gate electrodes, or a gate electrode when a multi-gatetransistor is considered as one transistor. That is, a component whichis formed of the same material as a gate electrode or a gate wire andconnected to the gate electrode or the gate wire may be referred to as agate electrode or a gate wire. Moreover, for example, a conductive filmof a portion which connects a gate electrode and a gate wire may bereferred to as a gate electrode or a gate wire.

It is to be noted that a gate terminal corresponds to a part of a regionof a gate electrode or a region electrically connected to a gateelectrode.

It is to be noted that a source includes a source region, a sourceelectrode, and a source wire (also referred to as source line, sourcesignal line, or the like), or a part of them. A source regioncorresponds to a semiconductor region which contains a lot of P-typeimpurities (boron, gallium, or the like) or N-type impurities(phosphorus, arsenic, or the like). Therefore, a region containing asmall amount of P-type impurities or N-type impurities, that is, an LDD(Lightly Doped Drain) region is not included in a source region. Asource electrode corresponds to a conductive layer of a part which isformed of a different material from a source region and electricallyconnected to the source region. However, a source electrode may bereferred to as a source electrode including a source region. A sourcewire corresponds to a wire for connecting source electrodes of pixelsand connecting a source electrode and another wire.

However, there is a part which functions as a source electrode and alsoas a source wire. Such a region may be referred to as a source electrodeor a source wire. That is, there is a region which cannot bespecifically determined as a source electrode or a source wire. Forexample, when there is a source region overlapping a source wire whichis extended, the region functions as a source wire and also as a sourceelectrode. Therefore, such a region may be referred to as a sourceelectrode or a source wire.

Further, a portion which is formed of the same material as a sourceelectrode and connected to the source electrode may be referred to as asource electrode as well. A portion which connects one source electrodeand another source electrode may also be referred to as a sourceelectrode as well. Further, a portion overlapping a source region may bereferred to as a source electrode. Similarly, a region which is formedof the same material as a source wire and connected to the source wiremay be referred to as a source wire. In a strict sense, such a regionmay not have a function to connect to another source electrode. However,there is a region which is formed of the same material as a sourceelectrode or a source wire and connected to a source electrode or asource wire due to a manufacturing margin and the like. Therefore, sucha region may also be referred to as a source electrode or a source wire.

For example, a conductive film of a portion which connects a sourceelectrode and a source wire may be referred to as a source electrode ora source wire.

It is to be noted that a source terminal corresponds to a part of asource region, a source electrode, or a region electrically connected toa source electrode.

It is to be noted that as for a drain, the similar thing to a source canbe applied.

It is to be noted in the invention that a semiconductor devicecorresponds to a device including a circuit having a semiconductorelement (transistor, diode, or the like). Further, a semiconductordevice may be a general device which can function by utilizingsemiconductor characteristics. A display device corresponds to a deviceincluding a display element (liquid crystal element, light emittingelement, or the like). It is to be noted that a display device may be amain body of a display panel in which a plurality of pixels includingdisplay elements such as a liquid crystal element and an EL element anda peripheral driver circuit for driving the pixels are formed over asubstrate. Further, a display device may include the one provided with aflexible printed circuit (FPC) or a printed wiring board (PWB) (IC,resistor, capacitor, inductor, transistor, or the like). Moreover, adisplay device may include an optical sheet such as a polarizing plateor a retardation film. In addition, a backlight (such as light guideplate, prism sheet, diffusion sheet, reflection sheet, light source(LED, cold-cathode tube, or the like) may be included. A light emittingdevice corresponds to a display device including a self-light emittingdisplay element such as an EL element or an element used for an FED inparticular. A liquid crystal display device corresponds to a displaydevice including a liquid crystal element.

It is to be noted in the invention that when it is described that anobject is formed on another object, it does not necessarily mean thatthe object is in direct contact with the another object. In the casewhere the above two objects are not in direct contact with each other,still another object may be interposed therebetween. Accordingly, whenit is described that a layer B is formed on a layer A, it means either acase where the layer B is formed in direct contact with the layer A, ora case where another layer (such as a layer C or a layer D) is formed indirect contact with the layer A, and then the layer B is formed indirect contact with the another layer. In addition, when it is describedthat an object is formed over or above another object, it does notnecessarily mean that the object is in direct contact with the anotherobject, and another object may be interposed therebetween. Accordingly,when it is described that a layer B is formed over or above a layer A,it means either a case where the layer B is formed in direct contactwith the layer A, or a case where another layer (such as a layer C or alayer D) is formed in direct contact with the layer A, and then thelayer B is formed in direct contact with the another layer. Similarly,when it is described that an object is formed below or under anotherobject, it means either a case where the objects are in direct contactwith each other or not in contact with each other.

The invention can provide a display device capable of preventing acurrent from flowing to a display element in signal writing operationwithout changing potentials of power source lines for supplying acurrent to the display element per row.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing a pixel structure of the invention.

FIG. 2 is a diagram showing a pixel structure of the invention.

FIGS. 3A to 3C are diagrams showing operation of a pixel of theinvention.

FIG. 4 is a diagram showing a display device of the invention.

FIG. 5 is a diagram showing a pixel structure of the invention.

FIG. 6 is a diagram showing a pixel structure of the invention.

FIGS. 7A to 7D are diagrams showing operation of a pixel of theinvention.

FIGS. 8A and 8B are diagrams showing operation of a pixel of theinvention.

FIGS. 9A and 9B are diagrams showing operation of a pixel of theinvention.

FIGS. 10A and 10B are diagrams showing operation of a pixel of theinvention.

FIG. 11 is a diagram showing a pixel structure of the invention.

FIGS. 12A to 12C are diagrams showing operation of a pixel of theinvention.

FIG. 13 is a diagram showing a pixel structure of the invention.

FIG. 14 is a diagram showing a pixel structure of the invention.

FIG. 15 is a diagram showing a pixel structure of the invention.

FIG. 16 is a diagram showing a pixel structure of the invention.

FIG. 17 is a diagram showing a pixel structure of the invention.

FIG. 18 is a diagram showing a pixel structure of the invention.

FIGS. 19A and 19B are diagrams showing a connecting state when a pixelof the invention operates.

FIG. 20 is a diagram showing a pixel structure of the invention.

FIG. 21 is a diagram showing a basic principle of the invention.

FIGS. 22A and 22B are diagrams showing a display panel of the invention.

FIGS. 23A and 23B are diagrams showing a light emitting elementapplicable to a display device of the invention.

FIGS. 24A to 24C are diagrams showing a display panel of the invention.

FIG. 25 is a diagram showing a display panel of the invention.

FIGS. 26A and 26B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 27A and 27B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 28A and 28B are diagrams showing display panels of the invention.

FIGS. 29 A and 29B are diagrams showing a display panel of theinvention.

FIGS. 30A and 30B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 31A and 31B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 32A and 32B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 33A and 33B are diagrams showing structures of a transistor and acapacitor applicable to a pixel of the invention.

FIGS. 34A to 34H are views showing electronic appliances to which adisplay device of the invention can be applied.

FIG. 35 is a view showing an example of an EL module.

FIG. 36 is a block diagram showing a main structure of an EL televisionreceiver.

FIG. 37 is a view showing an example of a structure of a mobile phone.

FIG. 38 is a diagram showing a pixel structure of the invention.

FIG. 39 is a diagram showing a pixel structure of the invention.

FIG. 40 is a diagram showing a driving method of the invention.

FIG. 41 is a diagram showing a pixel structure of the invention.

FIG. 42 is a diagram showing a pixel structure of the invention.

FIG. 43 is a cross sectional view of a part of a pixel.

FIG. 44 is a diagram showing a driving method of the invention.

FIG. 45 is a diagram showing a display device of the invention.

FIG. 46 is a diagram showing a pixel structure of the invention.

FIG. 47 is a diagram showing a pixel structure of the invention.

FIG. 48 is a diagram showing a pixel structure of the invention.

FIG. 49 is a diagram showing a pixel structure of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[Embodiment Mode]

Although the invention will be fully described by way of embodimentmodes and embodiments with reference to the accompanying drawings, it isto be understood that various changes and modifications will be apparentto those skilled in the art. Therefore, unless such changes andmodifications depart from the scope of the invention, they should beconstrued as being included therein.

The invention can be applied to not only a pixel including an EL elementand the like but various analog circuits each including a currentsource. First, in this embodiment mode, description is made of a basicprinciple of the invention.

First, FIG. 21 shows a structure of a semiconductor device based on abasic principle of the invention. The semiconductor device includes atransistor 2101, a first switch 2102, a second switch 2103, a capacitor2104, a load 2105, a first wire 2106, a second wire 2107, and a thirdwire 2108. It is to be noted that the transistor 2101 is an n-channeltransistor.

A connection structure of the semiconductor device is described.

A first terminal (one of a source terminal and a drain terminal) and asecond terminal (the other of the source terminal and the drainterminal) of the transistor 2101 are connected to the load 2105 and thesecond wire 2107 respectively. A gate terminal of the transistor 2101 isconnected to the third wire 2108 through the second switch 2103. That isto say, when the second switch 2103 is in an on state, the gate terminalof the transistor 2101 and the third wire 2108 are electricallyconnected. On the other hand, when the second switch 2103 is in an offstate, the gate terminal of the transistor 2101 and the third wire 2108are electrically disconnected.

The first terminal of the transistor 2101 is connected to the first wire2106 through the first switch 2102. That is to say, when the firstswitch 2102 is in an on state, the first terminal of the transistor 2101and the first wire 2106 are electrically connected. On the other hand,when the first switch 2102 is in an off state, the first terminal of thetransistor 2101 and the first wire 2106 are electrically disconnected.

The capacitor 2104 is connected between the gate terminal and the firstterminal of the transistor 2101. That is to say, a first electrode and asecond electrode of the capacitor 2104 are connected to the gateterminal and the first terminal of the transistor 2101 respectively. Itis to be noted that the capacitor 2104 may have a structure where aninsulating film is interposed between a wire, an active layer, anelectrode, and the like, or can be omitted by using the gate capacitanceof the transistor 2101.

It is to be noted that a predetermined potential is inputted to thesecond wire 2107 and the third wire 2108.

Subsequently, operation of the semiconductor device is described.

In setting operation, the first switch 2012 and the second switch 2103are turned on.

Then, a charge is accumulated in the capacitor 2104; therefore, acurrent flows to the transistor 2101. A current which flows at this timeis a current set to the first wire 2106.

When accumulating the charge in the capacitor 2104 is completed, thefirst switch 2102 and the second switch 2103 are turned off. Then, agate-source voltage of the transistor 2101 is held in the capacitor2104. Further, by adjusting a potential of the third wire 2108 at thistime, a current can be prevented from flowing to the load 2105.

It is to be noted that the gate-source voltage of the transistor 2101 isa voltage to apply the same amount of current as that flowing throughthe first wire 2106 to the transistor 2101.

In outputting operation, the first switch 2102 and the second switch2103 are turned off, and then the gate terminal of the transistor 2101is set in a floating state. A gate-source voltage of the transistor 2101is held in the capacitor 2104. Therefore, a current which has flown tothe first wire 2106 in the setting operation flows from the second wire2107 to the load 2105 through the transistor 2101

At this time, the first terminal of the transistor 2101 is a sourceterminal and has a higher potential. The drain-source voltage of thetransistor 2101 becomes lower than that in the setting operation.However, since the transistor 2101 is operated in a saturation region,almost the same current as that flowing through the first wire 2106 inthe setting operation can be applied to the load 2105.

It is to be noted that an n-channel transistor is used for thetransistor 2101. Alternatively, a p-channel transistor may be used aswell. In this case, direction of a current is opposite.

(Embodiment Mode 1)

In this embodiment mode, description is made of a basic pixel structurein the case of applying the invention to the pixel.

A pixel described in this embodiment mode includes a transistor 101, afirst switch 102, a second switch 103, a capacitor 104, a displayelement 105, a first wire 106, a second wire 107, a third wire 108, anda fourth wire 109. It is to be noted that the transistor 101 is ann-channel transistor.

A connection structure of the pixel is described.

A first terminal (one of a source terminal and a drain terminal) and asecond terminal (the other of the source terminal and the drainterminal) of the transistor 101 are connected to a pixel electrode ofthe display element 105 and the third wire 108 respectively. A gateterminal of the transistor 101 is connected to the fourth wire 109through the second switch 103. That is to say, when the second switch103 is in an on state, the gate terminal of the transistor 101 and thefourth wire 109 are electrically connected. On the other hand, when thesecond switch 103 is in an off state, the gate terminal of thetransistor 101 and the fourth wire 109 are electrically disconnected.

The first terminal of the transistor 101 is connected to the second wire107 through the first switch 102. That is to say, when the first switch102 is in an on state, the first terminal of the transistor 101 and thesecond wire 107 are electrically connected. On the other hand, when thefirst switch 102 is in an off state, the first terminal of thetransistor 101 and the second wire 107 are electrically disconnected.

The capacitor 104 is connected between the gate terminal and the firstterminal of the transistor 101. That is to say, a first electrode and asecond electrode of the capacitor 104 are connected to the gate terminaland the first terminal of the transistor 101 respectively. It is to benoted that the capacitor 104 may have a structure where an insulatingfilm is interposed between a wire, an active layer, an electrode, andthe like, or can be omitted by using the gate capacitance of thetransistor 101.

It is to be noted that a predetermined potential is inputted to anopposed electrode 110 of the display element 105, the third wire 108,and the fourth wire 109.

By inputting a signal to the first wire 106, the first switch 102 andthe second switch 103 are controlled to be turned on or off.

A signal is inputted to the second wire 107 in accordance with a grayscale level of the pixel. This signal corresponds to a video signal anda signal current flows to the second wire 107.

It is to be noted that transistors can be applied to the first switch102 and the second switch 103. FIG. 2 shows the case of applyingn-channel transistors to the first switch 102 and the second switch 103.Note that common portions to those in FIG. 1 are denoted by the samereference numerals, and description thereof is omitted.

A first switching transistor 201 corresponds to the first switch 102 anda second switching transistor 202 corresponds to the second switch 103.

A gate terminal, a first terminal (one of a source terminal and a drainterminal), and a second terminal (the other of the source terminal andthe drain terminal) of the first switching transistor 201 are connectedto the first wire 106, the second wire 107, and a pixel electrode of thedisplay element 105 and the first terminal of the transistor 101,respectively. Therefore, when a signal inputted to the first wire 106 isat H level, the first switching transistor 201 is turned on whereas whenthe signal is at L level, the first switching transistor 201 is turnedoff.

A gate terminal, a first terminal (one of a source terminal and a drainterminal), and a second terminal (the other of the source terminal andthe drain terminal) of the second switching transistor 202 are connectedto the first wire 106, the gate terminal of the transistor 101, and thefourth wire 109 respectively. Therefore, when a signal inputted to thefirst wire 106 is at H level, the second switching transistor 202 isturned on whereas when the signal is at L level, the second switchingtransistor 202 is turned off.

Subsequently, description is made with reference to FIGS. 3A to 3C ofoperation of the pixel of this embodiment mode. Note that in FIGS. 3A to3C, description is made by using the pixel structure in FIG. 2 since thepixels of FIGS. 1 and 2 operate in the same manner.

It is to be noted that a current source 301 connected to the second wire107 sets a signal current Idata which is written to the pixel. Thesecond wire 107 is connected to a wire 302 through a current source 301.A predetermined potential is inputted to the wire 302. Here, potentialsinputted to the third wire 108, the fourth wire 109, the wire 302, andthe opposed electrode 110 are denoted by V3, V4, V5, and Vcomrespectively. As for a relation of the potentials, V3>Vcom>V5 is atleast satisfied. When V4=Vcom is satisfied, the fourth wire 109 and theopposed electrode 110 of the display element 105 may be connectedthrough a fifth wire 4801 as shown in FIG. 48.

It is to be noted that the operation of a pixel includes signal writingoperation for writing a signal to a pixel and light emitting operationfor emitting light of a gray scale level in accordance with a signalwritten to a pixel. FIGS. 3A and 3B are diagrams showing the signalwriting operation, and FIG. 3C is a diagram showing the light emittingoperation.

First, a transient state in the signal writing operation is describedwith reference to FIG. 3A. A signal to be inputted to the first wire 106is set to be at H level, thereby turning on the first and secondswitching transistors 201 and 202. Accordingly, a current flows as shownin FIG. 3A. That is, as paths of current, there are a first path where acurrent flows from the fourth wire 109 to the capacitor 104 through thesecond switching transistor 202 and a second path where a current flowsfrom the third wire 108 to the transistor 101. A current Ic that flowsthrough the first path and a current Itr that flows through the secondpath unite at a connecting portion of the first terminal of thetransistor 101 and the second electrode of the capacitor 104. Then, acurrent Ic and a current Itr flow as the signal current Idata to thewire 302 through the first switching transistor 201 and a current source301. That is to say, Ic+Itr=Idata is satisfied.

A current does not flow to the capacitor 104 before long, which leads toa steady state in the signal writing operation. Therefore, a currentflows as shown in FIG. 3B. A current Itr that flows from the third wire108 to the transistor 101 is equal to the signal current Idata. That is,a gate-source voltage Vgs of the transistor 101 is necessary forapplying the signal current Idata to the transistor 101. A load for thegate-source voltage Vgs of the transistor 101 is accumulated in thecapacitor 104.

It is to be noted that when potentials of the gate terminal and thefirst terminal of the transistor 101 at this time are denoted by Va andVb respectively, Vgs=(Va−Vb) is satisfied. When a forward thresholdvoltage of the display element 105 is denoted by V_(ELth),(Vb−Vcom)<V_(ELth) is preferably satisfied, thereby applying no currentto the display element 105 in the signal writing operation. Therefore,the potential V4 inputted to the fourth wire 109 is desirably set so asto satisfy V3>V4>V5. When V4=Vcom is satisfied, the number of powersources necessary for pixels can be reduced. Further, a reverse bias canbe applied to the display element 105 in the signal writing operation.

It is to be noted that even when a reverse bias is applied to thedisplay element 105, a current does not flow to the display element 105normally (if flows, it is a small amount of current). On the other hand,if the display element 105 is short-circuited, a current flows to ashort-circuited portion. Then, the short-circuited portion is insulated,thereby a display defect can be improved.

Subsequently, description is made with reference to FIG. 3C of the lightemitting operation. A signal inputted to the first wire 106 is set to beat an L level, thereby turning off the first and second switchingtransistors 201 and 202. Thus, a current flows as shown in FIG. 3C. Atthis time, the second switching transistor 202 is in an off state.Therefore, the capacitor 104 holds the gate-source voltage Vgs necessaryfor applying the signal current Idata to the transistor 101.Accordingly, a current which is almost equivalent to the signal currentIdata flows to the transistor 101.

It is to be noted that when potentials of the gate terminal and thefirst terminal of the transistor 101 at this time are denoted by Va′ andVb′ respectively, Vgs=(Va′−Vb′) is satisfied. This is because Va′ isincreased as Vb′ is increased since the capacitor 104 holds thegate-source voltage Vgs although Vb′>Vb is satisfied.

It is to be noted that when potentials of H level signal and an L levelsignal to be inputted to the first wire 106 are denoted by V1(H) andV1(L) respectively, the following potentials are preferable. Thresholdvoltages of the first switching transistor 201 and the second switchingtransistor 202 are denoted by Vth1 and Vth2 respectively.

As shown in FIG. 3B, even when a potential of the pixel electrode of thedisplay element 105 becomes Vb, the first switching transistor 201 isrequired to be in an on state. Therefore, V1(H)>(Vb+Vth1) is set to besatisfied. Further, V1(H)>(V4+Vth2) is set to be satisfied so that thesecond switching transistor 202 is in an on state. Specifically, forexample, when V4=Vcom is satisfied, V1(H) is preferably a potentialhigher than Vcom by 1 to 8 V.

As shown in FIG. 3C, V1(L)<(Vb+Vth1) is satisfied so that the firstswitching transistor 201 is turned off. That is, when the signal currentis written to another pixel, a potential of the second wire 107 becomesVb. Therefore, in a pixel which is not selected at this time, the firstswitching transistor 201 is required to be in an off state. On the otherhand, V1(L)<(V4+Vth2) is satisfied so that the second switchingtransistor 202 is in an off state. Specifically, for example, whenV4=Vcom is satisfied, V1(L) is preferably a potential lower than Vcom by1 to 8 V.

By employing a pixel structure described in this embodiment mode, apotential of a gate terminal of a transistor in the signal writingoperation is controlled, thereby preventing a current from flowing to adisplay element at this time.

It is to be noted that by employing the pixel structure shown in FIG. 2,a pixel can be formed of only n-channel transistors, which can simplifymanufacturing steps. An amorphous semiconductor, a semi-amorphoussemiconductor (also referred to as a microcrystalline semiconductor), orthe like can be used for a semiconductor layer of a transistorconstituting a pixel. For example, amorphous silicon (a-Si:H) may beused as the amorphous semiconductor. Therefore, the manufacturing stepscan be further simplified. As a result, reduction in a manufacturingcost and improvement in the yield can be achieved.

Further, by employing the structure of the invention, Vds>Vgs can besatisfied in the signal writing operation. A change in the Vds can bemade small between in the signal writing operation and in the lightemitting operation. Therefore, even if constant current characteristics(flatness of current) in a saturation region of the transistor 101 arebad, current values are almost equivalent between in the signal writingoperation and in the light emitting operation. In particular, when anamorphous semiconductor film (such as amorphous silicon) is used as asemiconductor layer of the transistor 101, constant currentcharacteristics (flatness of current) in a saturation region of thetransistor 101 may be deteriorated. Therefore, when the structure of theinvention is applied in the case where an amorphous semiconductor filmis used as a semiconductor layer of the transistor 101, a display defectcan be prevented.

Further, since a high voltage is applied between the source and drainterminals of the transistor 101 shown in FIG. 2, the channel length ofthe transistor 101 may be longer than that of the first switchingtransistor 201 or the second switching transistor 202. Alternatively, amulti-gate transistor may be applied to the transistor 101 as shown inFIG. 16. Accordingly, the pressure resistance of the transistor isincreased, thereby preventing the transistor from being damaged.

Further, in order to apply a current controlled by the transistor 101shown in FIG. 2 to the display element 105, the transistor 101 isrequired to have a capability of applying a large amount of on current.Therefore, the channel width of the transistor 101 may be wider thanthat of the first switching transistor 201 or the second switchingtransistor 202. Alternatively, the transistor 101 may have a structurewhere a plurality of transistors are connected in parallel as atransistor 1701 shown in FIG. 17.

Subsequently, description is made with reference to FIG. 4 of a displaydevice including a pixel of the invention.

A display device includes a signal line driver circuit 401, a scan linedriver circuit 402, and a pixel portion 403. The pixel portion 403includes a plurality of signal lines S1 to Sn extended in columndirection from the signal line driver circuit 401, a plurality of scanlines G1 to Gm extended in row direction from the scan line drivercircuit 402, and a plurality of pixels 404 arranged in matrixcorresponding to the signal lines S1 to Sn and the scan lines G1 to Gm.Further, the pixel portion 403 includes power source lines P1 to Pn andbias lines B1 to Bn which are parallel to the signal lines S1 to Sn.Each of the pixels 404 is connected to a signal line Sj (any one of thesignal lines S1 to Sn), a scan line Gi (any one of the scan lines G1 toGm), a power source line Pj (any one of the power source lines P1 toPn), and a bias line Bj (any one of the bias lines B1 to Bn).

It is to be noted that the scan line Gi corresponds to the first wire106 in FIG. 1. The signal line Sj corresponds to the second wire 107 inFIG. 1. The power source line Pj corresponds to the third wire 108 inFIG. 1. The bias line Bj corresponds to the fourth wire 109 in FIG. 1.

The scan lines G1 to Gm are selected one by one by a signal outputtedfrom the scan line driver circuit 402. Then, the signal is written tothe pixel 404 connected to the scan line which is selected. At thistime, a signal current flows to each of the signal line S1 to Sn inaccordance with a gray scale level of each pixel.

After signal writing is completed, another scan line is selected, andthen signal writing is performed to the pixel 404 connected to the scanline. The pixel to which a signal has been written starts light emittingoperation and emits light in accordance with the signal written to thepixel. Thus, signals are sequentially written to the pixels 404 toperform signal writing is all the pixels 404 sequentially.

However, the structure of the display device shown in FIG. 4 is oneexample and the invention is not limited to this. That is, the powersource lines P1 to Pn and the bias lines B1 to Bn are not required to bearranged parallel to the signal lines S1 to Sn. The power source linesand the bias lines may be arranged parallel to the scan lines G1 to Gm.Alternatively, each of the power source lines and the bias lines may bearranged in a grid pattern. It is to be noted that in the case where thepixel portion 403 includes a plurality of color elements, the powersource lines and the bias lines are preferably arranged as shown in FIG.4.

That is to say, the fourth wire 109 in the pixel of FIG. 1 may bearranged parallel to the first wire 106 as shown in FIG. 46. In thiscase, bias lines B1 to Bm corresponding to the bias lines B1 to Bn inFIG. 4 are arranged parallel to the scan lines G1 to Gm as shown in FIG.47. Potentials of the bias lines B1 to Bm may be varied. In other words,the bias lines may be scanned. In this case, a bias line driver circuitmay be provided in addition to the scan line driver circuit 402 whichscans the scan lines G1 to Gm.

In the case where the pixel portion 403 includes a plurality of colorelements, potentials of a power source line and a bias line connected toeach pixel which is a color element may be varied. Further, the size ofa pixel electrode may be different per pixel to be a color element. Inother words, a light emitting area may be different per pixel to be acolor element. Thus, in the case where an EL element of a differentcolor is used as a display element for a full color display, a balanceof colors and a progress of deterioration of the EL element can becontrolled.

A pixel of the invention is not limited to the structure of FIG. 1. Apixel is only required to be connected as shown in FIG. 19A in signalwriting operation whereas connected as shown in FIG. 19B in lightemitting operation. That is to say, in the signal writing operation, thegate terminal, the first terminal, and the second terminal of thetransistor 101 are only required to be connected to the fourth wire 109,the second wire 107, and the third wire 108 respectively. On the otherhand, in the light emitting operation, it is only required that the gateterminal of the transistor 101 be electrically connected nowhere and thefirst terminal and the second terminal of the transistor 101 beconnected to the pixel electrode of the display element 105 and thethird wire 108 respectively.

Thus, in the pixel shown in FIG. 1, an additional wire may be providedto control on/off of the first switch 102 and the second switch 103separately. That is to say, a fifth wire 501 for controlling on/off ofthe second switch 103 may be provided in addition to the first wire 106for controlling on/off of the first switch 102. In this case, after thesignal writing operation is completed, the first switch 102 and thesecond switch 103 are turned off at the same time or the second switch103 is turned off before the first switch 102 is turned off. If thesecond switch 103 is in an on state even after the first switch 102 isturned off, a charge accumulated in the capacitor 104 is dischargedthrough the transistor 101.

In the case of a structure shown in FIG. 5, when V4=Vcom is satisfied,the fourth wire 109 and the opposed electrode 110 of the display element105 may be connected through a sixth wire 4901 as shown in FIG. 49.

In the pixel of FIG. 1 or FIG. 2, the first wire 106 in a pixel ofanother row can be used instead of the fourth wire 109. That is, in thiscase, the bias lines B1 to Bn of the display device shown in FIG. 4 canbe omitted. As an example, FIG. 13 shows a structure where the fourthwire 109 in the pixel of FIG. 2 is omitted and the first wire 106 in apixel of the adjacent row is used instead of the fourth wire 109.

As shown in FIG. 14, the first switching transistor 201 and the secondswitching transistor 202 which are n-channel transistors can be appliedto the first switch 102 and the second switch 103 in the pixel of FIG. 5respectively, and the fifth wire 501 in a pixel of another row can beused instead of the fourth wire 109.

As shown in FIG. 15, the first switching transistor 201 and the secondswitching transistor 202 which are n-channel transistors can be appliedto the first switch 102 and the second switch 103 in the pixel of FIG. 5respectively, and the first wire 106 in a pixel of another row can alsobe used instead of the fourth wire 109.

As shown in FIG. 20, a first switching transistor 2001 and a secondswitching transistor 2002 which are p-channel transistors can be appliedto the first switch 102 and the second switch 103 in the pixel of FIG. 1respectively, and the first wire 106 in a pixel of another row can beused instead of the third wire 108.

(Embodiment Mode 2)

When a pixel is formed using a transistor, variation in characteristicsof transistors in different pixels is a problem. The variation intransistor characteristics is recognized as display unevenness.

In this embodiment mode, description is made of a case where transistors(transistors to be turned on) used in pixels of the invention areswitched per period, thereby transistor characteristics can be averagedin terms of time and display unevenness can be hardly recognized.

A pixel of this embodiment mode is shown in FIG. 6.

A pixel of this embodiment mode includes a first transistor 601, asecond transistor 611, a first switch 602, a second switch 603, a thirdswitch 612, a fourth switch 613, a capacitor 604, a display element 605,a first wire 606, a second wire 607, a third wire 608, and a fourth wire609. It is to be noted that the first transistor 601 and the secondtransistor 611 are n-channel transistors.

First, a connection structure of the pixel is described.

A first terminal (one of a source terminal and a drain terminal) of thefirst transistor 601 is connected to a pixel electrode of the displayelement 605, a second terminal (the other of the source terminal and thedrain terminal) of the first transistor 601 is connected to the thirdwire 608 through the third switch 612, and a gate terminal of the firsttransistor 601 is connected to the fourth wire 609 through the secondswitch 603. That is to say, when the third switch 612 is in an on state,the second terminal of the first transistor 601 and the third wire 608are electrically connected. On the other hand, when the third switch 612is in an off state, the second terminal of the first transistor 601 andthe third wire 608 are electrically disconnected. Further, when thesecond switch 603 is in an on state, the gate terminal of the firsttransistor 601 and the fourth wire 609 are electrically connected. Onthe other hand, when the second switch 603 is in an off state, the gateterminal of the first transistor 601 and the fourth wire 609 areelectrically disconnected.

Similarly, the second transistor 611 and the first transistor 601 areconnected in parallel. That is, a first terminal (one of a sourceterminal and a drain terminal) of the second transistor 611 is connectedto a pixel electrode of the display element 605, a second terminal (theother of the source terminal and the drain terminal) of the secondtransistor 611 is connected to the third wire 608 through the fourthswitch 613, and a gate terminal of the second transistor 611 isconnected to the fourth wire 609 through the second switch 603. That isto say, when the fourth switch 613 is in an on state, the secondterminal of the second transistor 611 and the third wire 608 areelectrically connected. On the other hand, when the fourth switch 613 isin an off state, the second terminal of the second transistor 611 andthe third wire 608 are electrically disconnected. Further, when thesecond switch 603 is in an on state, the gate terminal of the secondtransistor 611 and the fourth wire 609 are electrically connected. Onthe other hand, when the second switch 603 is in an off state, the gateterminal of the second transistor 611 and the fourth wire 609 areelectrically disconnected.

The first terminal of the first transistor 601 and the first terminal ofthe second transistor 611 are connected to the second wire 607 throughthe first switch 602. That is to say, when the first switch 602 is in anon state, the first terminals of the first transistor 601 and the secondtransistor 611 are electrically connected to the second wire 607. On theother hand, when the first switch 602 is in an off state, the firstterminals of the first transistor 601 and the second transistor 611 areelectrically disconnected to the second wire 607.

The gate terminals of the first transistor 601 and the second transistor611 are electrically connected, and the capacitor 604 is connectedbetween the gate terminals and the first terminals of the firsttransistor 601 and the second transistor 611. That is to say, a firstelectrode of the capacitor 604 is connected to the gate terminals of thefirst transistor 601 and the second transistor 611, and a secondelectrode of the capacitor 604 is connected to the first terminals ofthe first transistor 601 and the second transistor 611. It is to benoted that the capacitor 604 may have a structure where an insulatingfilm is interposed between a wire, an active layer, an electrode, andthe like, or can be omitted by using the gate capacitance of the firsttransistor 601 or the gate capacitance of the second transistor 611.

It is to be noted that a predetermined potential is inputted to anopposed electrode 610 of the display element 605, the third wire 608,and the fourth wire 609.

By inputting a signal to the first wire 606, the first switch 602 andthe second switch 603 are controlled to be turned on or off.

A signal is inputted to the second wire 607 in accordance with a grayscale level of the pixel. This signal corresponds to a video signal anda signal current flows to the second wire 607.

It is to be noted that transistors can be applied to the first switch602, the second switch 603, the third switch 612, and the fourth switch613. Therefore, n-channel transistors can also be applied to the firstswitch 602 and the second switch 603.

Subsequently, operation of the pixel of FIG. 6 is described.

It is to be noted that the operation of a pixel includes signal writingoperation for writing a signal to the pixel and light emitting operationfor emitting light of a gray scale level in accordance with a signalwritten to the pixel. Transistors (transistors to be turned on) used inpixels described in this embodiment mode are switched between in thesignal writing operation and in the light emitting operation in oneperiod and in the signal writing operation and in the light emittingoperation in another period.

FIG. 7A is a diagram showing signal writing operation in a certainperiod and FIG. 7B is a diagram showing light emitting operation in thisperiod. Further, FIG. 7C is a diagram showing signal writing operationin another period and FIG. 7D is a diagram showing light emittingoperation in this period. It is to be noted that a current source 701connected to the second wire 607 sets a signal current to be written tothe pixel. The second wire 607 is connected to a wire 702 through acurrent source 701. A predetermined potential is inputted to the wire702. Here, potentials inputted to the third wire 608, the fourth wire609, the wire 702, and the opposed electrode 610 are denoted by V3, V4,V5, and Vcom respectively. As for a relation of potentials, V3>Vcom>V5is at least satisfied.

Further, FIG. 7A shows a state where a pixel becomes a steady state inthe signal writing operation in a certain period and a current flow atthis time. The first switch 602, the second switch 603, and the fourthswitch 613 are in an on state whereas the third switch 612 is in an offstate. In this case, the second transistor 611 is used. That is to say,the signal current Idata set by a current source 701 flows from thethird wire 608 to the second transistor 611 through the fourth switch613. At this time, the second transistor 611 has a gate-source voltagehigh enough to apply the signal current Idata, and a charge for thevoltage is accumulated in the capacitor 604.

Therefore, in the light emitting operation, the first switch 602, thesecond switch 603, and the third switch 612 are turned off whereas thefourth switch 613 is turned on, and a current flows as shown in FIG. 7B.That is, a current flows from the third wire 608 to the display element605 through the fourth switch 613 and the second transistor 611. Thiscurrent is approximately equal to the signal current Idata.

However, the drain-source voltage of the second transistor 611 variesbetween in the signal writing operation and in the light emittingoperation, which generates a slight difference in amount of currentwhich flows to the second transistor 611. If there is a variation incharacteristics of the second transistor 611 per pixel, it is recognizedas display unevenness.

Thus in another period, in the signal writing operation, the firstswitch 602, the second switch 603, and the third switch 612 are turnedon whereas the fourth switch 613 is turned off. FIG. 7C shows a statewhere a pixel becomes a steady state in this period and a current flowat this time. In this case, the first transistor 601 is used. That is tosay, the signal current Idata set by a current source 701 flows from thethird wire 608 to the first transistor 601 through the third switch 612.At this time, the first transistor 601 has a gate-source voltage highenough to apply the signal current Idata, and a charge for the voltageis accumulated in the capacitor 604.

Therefore, in the light emitting operation, the first switch 602, thesecond switch 603, and the fourth switch 613 are turned off whereas thethird switch 612 is turned on, and a current flows as shown in FIG. 7D.That is, a current flows from the third wire 608 to the display element605 through the third switch 612 and the first transistor 601. Thiscurrent is approximately equal to the signal current Idata.

In this manner, transistors to be used are switched per period, therebytransistor characteristics can be averaged in terms of time.Accordingly, display unevenness can be reduced.

Further, another driving method can be applied to a pixel described inthis embodiment mode. For example, in signal writing operation, a signalis written with a large amount of signal current, and the amount ofcurrent applied to a display element in light emitting operation isreduced. Hereinafter, such a driving method is described

FIG. 8A is a diagram showing signal writing operation and FIG. 8B is adiagram showing light emitting operation.

Further, FIG. 8A shows a state where a pixel becomes a steady state inthe signal writing operation and a current flow at this time. The firstswitch 602, the second switch 603, the third switch 612, and the fourthswitch 613 are in an on state, and a current flows as shown in FIG. 8A.That is, as paths of current, there are a first path where a currentflows from the third wire 608 to the first transistor 601 through thethird switch 612 and a second path where a current flows from the thirdwire 608 to the second transistor 611 through the fourth switch 613. Acurrent I1 that flows through the first path and a current I2 that flowsthrough the second path unite at a connecting portion of the firstterminals of the first transistor 601 and the second transistor 611.Then, a current I1 and a current I2 flow as the signal current Idata tothe wire 702 through the first switch 602 and a current source 701. Thatis to say, I1+I2=Idata is satisfied.

Description is made with reference to FIG. 8B of the light emittingoperation. The first switch 602, the second switch 603, and the fourthswitch 613 are turned off whereas the third switch 612 is turned on, andthen a current flows as shown in FIG. 8B. Since the second switch 603 isin an off state at this time, the capacitor 604 holds a gate-sourcevoltage Vgs necessary for a current flowing to the first transistor 601and the second transistor 611 to be the signal current Idata.Accordingly, a current flows to the display element 605 through thefirst transistor 601. With this structure, this current can be adjusted.

Here, the channel length and the channel width of a transistor aredenoted by L and W respectively. When the transistor operates in asaturation region, a current value flowing through the transistor isgenerally proportional to W/L as far as a gate-source voltage isconstant. In other words, a current value is proportional to the channelwidth W and inversely proportional to the channel length L.

Therefore, the channel width of the first transistor 601 and the channelwidth of the second transistor 611 are denoted by W1 and W2respectively, and these transistors have the same channel length. If thefirst transistor 601 and the second transistor 611 through which acurrent flows are regarded as one transistor in FIG. 8A, the channelwidth and the channel length can be regarded as (W1+W2) and Lrespectively. On the other hand, in FIG. 8B, a current flows onlythrough the first transistor 601 and the transistor has the channelwidth W1 and the channel length L. Therefore, in light emittingoperation, a current of Idata×(W1/(W1+W2)) can be applied to the displayelement 605.

In this manner, the channel width or the channel length of the firsttransistor 601 or the second transistor 611 is adjusted, thereby asmaller amount of current than the signal current which is applied insignal writing operation can be applied to the display element 605.

Further, the channel width W1 and the channel length W2 are set to bethe same and a transistor used in light emitting operation is switchedper certain period. Accordingly, characteristics of the transistor canbe averaged in terms of time.

By switching the transistor to be used between in the signal writingoperation and in the light emitting operation, a ratio W/L of thechannel width W to the channel length L of the transistor which is usedin the signal writing operation and the light emitting operation may beadjusted and the amount of current applied to the display element may beadjusted.

That is, as shown in FIG. 9A, in the signal writing operation, the firstswitch 602, the second switch 603, and the fourth switch 613 are turnedon whereas the third switch 612 is turned off. Then, the signal currentIdata is applied from the third wire 608 to the second transistor 611through the fourth switch 613. In the light emitting operation, thefirst switch 602, the second switch 603, and the fourth switch 613 areturned off whereas the third switch 612 is turned on. Then, a current ofIdata×(W1/W2) flows through the first transistor 601. It is to be notedthat the amount of current applied to the display element 605 in thelight emitting operation can be set smaller than the signal currentIdata as far as W1<W2 is satisfied.

Thus, by writing a signal with a large amount of current in the signalwriting operation, even when parasitic capacitance is formed in the pathwhere the signal current flows, signal writing can be performed rapidly.Accordingly, a display defect can be prevented.

The above description is made of a case where the amount of currentwhich is applied to the display element in the light emitting operationis smaller than the signal current applied in the signal writingoperation. However, the amount of current which is applied to thedisplay element in the light emitting operation may be larger than thesignal current applied in the signal writing operation depending oncases. For example, in the signal writing operation, a current may beapplied to either the first transistor 601 or the second transistor 611whereas in the light emitting operation, a current may be applied toboth the first transistor 601 and the second transistor 611. It is to benoted in FIG. 9 that the amount of current applied to the displayelement 605 in the light emitting operation can be set larger than thesignal current Idata as far as W1>W2 is satisfied.

Further, in a pixel of this embodiment mode, pre-charging operation maybe performed. The operation is described with reference to FIG. 10. Inthis case, a current source 701 is connected to the second wire 607through a fifth switch 1003. The second wire 607 is connected to a wire1002 through a sixth switch 1004 and a pre-charging current source 1001.It is to be noted that the pre-charging current source 1001 to be usedwhich can set a larger amount of current than a current source 701. Apredetermined potential is inputted to the wire 1002. As the wire 702and the wire 1002, the same wire or different wires may be used.

First, FIG. 10A shows a state where a pixel becomes a steady state inthe pre-charging operation and a current flow at this time. The firstswitch 602, the second switch 603, the third switch 612, the fourthswitch 613, and the sixth switch 1004 are turned on whereas the fifthswitch 1003 is turned off. Then, a current set by the pre-chargingcurrent source 1001 flows from the third wire 608 to the firsttransistor 601 and the second transistor 611 through the third switch612 and the fourth switch 613, respectively. Thus, a charge isaccumulated in the capacitor 604.

In the setting operation, the first switch 602, the second switch 603,the third switch 612, and the fifth switch 1003 are turned on whereasthe fourth switch 613 and the sixth switch 1004 are turned off. Then, ina steady state, a current flows as shown in FIG. 10B. That is to say,the signal current Idata set by a current source 701 flows from thethird wire 608 to the first transistor 601. Then, a charge for thegate-source voltage necessary for applying the signal current Idata tothe first transistor 601 is accumulated in the capacitor 604.

A current applied to the pre-charging current source 1001, the channellength L1 and the channel width W1 of the first transistor 601, and thechannel length L2 and the channel width W2 of the second transistor 611are appropriately determined, thereby a charge which is accumulated inthe capacitor 604 in the pre-charging operation can be set so as to beapproximately equal to that in the setting operation, and the signalcurrent can be written to a pixel rapidly.

In FIG. 10, although a current is applied to the first transistor 601and the second transistor 611 in the pre-charging operation, a currentmay be applied to only one of them. Then, in the setting operation, acurrent may be applied to the other transistor.

As described above, the invention is not limited to a structure wherethe third switch 612 is connected between a second terminal of the firsttransistor 601 and the third wire 608, and the fourth switch 613 isconnected between a second terminal of the second transistor 611 and thethird wire 608. A structure shown in FIG. 18 may be employed. That is, afirst terminal (one of a source terminal and a drain terminal) of thefirst transistor 601 is connected to a pixel electrode of the displayelement 605 through a third switch 1801 and a second terminal (the otherof the source terminal and the drain terminal) of the first transistor601 is connected to the third wire 608. That is to say, when the thirdswitch 1801 is in an on state, the first terminal of first transistor601 and the pixel electrode of the display element 605 are electricallyconnected. On the other hand, when the third switch 1801 is in an offstate, the first terminal of the first transistor 601 and the pixelelectrode of the display element 605 are electrically disconnected.Similarly, the second transistor 611 is connected to the firsttransistor 601 in parallel. That is, first terminal (one of a sourceterminal and a drain terminal) of the second transistor 611 is connectedto a pixel electrode of the display element 605 through a fourth switch1802 and a second terminal (the other of the source terminal and thedrain terminal) of the second transistor 611 is connected to the thirdwire 608. That is to say, when the fourth switch 1802 is in an on state,the first terminal of the second transistor 611 and the pixel electrodeof the display element 605 are electrically connected. On the otherhand, when the fourth switch 1802 is in an off state, the first terminalof the second transistor 611 and the pixel electrode of the displayelement 605 are electrically disconnected.

In this embodiment mode, in the signal writing operation, a gateterminal of a transistor to which a current is applied can be set so asto have a predetermined potential; therefore, a potential differencebetween the pixel electrode of the display element and the opposedelectrode can be lower than a forward threshold voltage of the displayelement. Accordingly, a current can be prevented from flowing to thedisplay element in the signal writing operation.

Also in this embodiment mode, n-channel transistors may be used for thefirst switch 602, the second switch 603, the third switch 612, and thefourth transistor 613, thereby a pixel can be formed of a unipolartransistor. Accordingly, manufacturing steps can be simplified. As aresult, reduction in a manufacturing cost and improvement in the yieldcan be achieved. Further, since a pixel can be formed of only ann-channel transistor, a semiconductor layer of the transistor which isincluded in the pixel can be formed of an amorphous semiconductor, asemi-amorphous semiconductor (also referred to as microcrystallinesemiconductor), or the like. For example, amorphous silicon (a-Si:H) maybe used as an amorphous semiconductor. Therefore, manufacturing stepscan be further simplified. As a result, reduction in a manufacturingcost and improvement in the yield can be achieved.

(Embodiment Mode 3)

In this embodiment mode, description is made with reference to FIG. 11of a case where a p-channel transistor is applied to a transistorincluded in a pixel of the invention.

A pixel described in this embodiment mode includes a transistor 1101, afirst switching transistor 1102, a second switching transistor 1103, acapacitor 1104, a display element 1105, a first wire 1106, a second wire1107, a third wire 1108, and a fourth wire 1109. It is to be noted thatthe transistor 1101, the first switching transistor 1102, and the secondswitching transistor 1103 are p-channel transistors.

First, a connection structure of the pixel is described.

A first terminal (one of a source terminal and a drain terminal) and asecond terminal (the other of the source terminal and the drainterminal) of the transistor 1101 are connected to a pixel electrode ofthe display element 1105 and the third wire 1108 respectively. A gateterminal of the transistor 1101 is connected to the fourth wire 1109through the second switching transistor 1103. That is to say, when thesecond switching transistor 1103 is in an on state, the gate terminal ofthe transistor 1101 and the fourth wire 1109 are electrically connected.On the other hand, when the second switching transistor 1103 is in anoff state, the gate terminal of the transistor 1101 and the fourth wire1109 are electrically disconnected. A gate terminal, a first terminal(one of a source terminal and a drain terminal), and a second terminal(the other of the source terminal and the drain terminal) of the secondswitching transistor 1103 are connected to the first wire 1106, the gateterminal of the transistor 1101, and the fourth wire 1109 respectively.Therefore, when a signal inputted to the first wire 1106 is at H level,the second switching transistor 1103 is turned on whereas when thesignal is at L level, the second switching transistor 1103 is turnedoff.

Further, the first terminal of the transistor 1101 is connected to thesecond wire 1107 through the first switching transistor 1102. That is tosay, when the first switching transistor 1102 is in an on state, thefirst terminal of the transistor 1101 and the second wire 1107 areelectrically connected. On the other hand, when the first switchingtransistor 1102 is in an off state, the first terminal of the transistor1101 and the second wire 1107 are electrically disconnected. A gateterminal, a first terminal (one of a source terminal and a drainterminal), and a second terminal (the other of the source terminal andthe drain terminal) of the first switching transistor 1102 are connectedto the first wire 1106, the second wire 1107, and the pixel electrode ofthe display element 1105 and the first terminal of the transistor 1101,respectively. Therefore, when a signal inputted to the first wire 1106is at H level, the first switching transistor 1102 is turned on whereaswhen the signal is at L level, the first switching transistor 1102 isturned off.

The capacitor 1104 is connected between the gate terminal and the firstterminal of the transistor 1101. That is to say, a first electrode and asecond electrode of the capacitor 1104 are connected to the gateterminal and the first terminal of the transistor 1101 respectively. Itis to be noted that the capacitor 1104 may have a structure where aninsulating film is interposed between a wire, an active layer, anelectrode, and the like, or can be omitted by using the gate capacitanceof the transistor 1101.

It is to be noted that a predetermined potential is inputted to theopposed electrode 1110 of the display element 1105, the third wire 1108,and the fourth wire 1109.

By inputting a signal to the first wire 1106, the first switchingtransistor 1102 and the second switching transistor 1103 are controlledto be turned on or off.

A signal is inputted to the second wire 1107 in accordance with a grayscale level of a pixel. This signal corresponds to a video signal and asignal current flows to the second wire 1107.

Subsequently, description is made with reference to FIGS. 12A to 12C ofoperation of the pixel of this embodiment mode.

It is to be noted that a current source 1201 connected to the secondwire 1107 sets a signal current Idata which is written to a pixel. Thesecond wire 1107 is connected to a wire 1202 through a current source1201. A predetermined potential is inputted to the wire 1202. Here,potentials inputted to the third wire 1108, the fourth wire 1109, thewire 1202, and the opposed electrode 1110 are denoted by V3, V4, V5, andVcom respectively. As for a relation of the potentials, V3<Vcom<V5 is atleast satisfied.

It is to be noted that the operation of a pixel includes signal writingoperation for writing a signal to a pixel and light emitting operationfor emitting light of a gray scale level in accordance with the signalwritten to the pixel. FIGS. 12A and 12B are diagrams showing signalwriting operation, and FIG. 12C is a diagram showing light emittingoperation.

First, a transient state in signal writing operation is described withreference to FIG. 12A. A signal which is inputted to the first wire 1106is set to be at L level, thereby turning on the first and secondswitching transistors 1102 and 1103. Accordingly, a current flows asshown in FIG. 12A. That is, the signal current Idata set by a currentsource 1201 flows to the capacitor 1104 and the transistor 1101. If acurrent Ic and a current Itr flow to the capacitor 1104 and thetransistor 1101 respectively, Ic+Itr=Idata is satisfied.

A current does not flow to the capacitor 1104 before long, which leadsto a steady state in the signal writing operation. Therefore, a currentflows as shown in FIG. 12B. A current Itr that flows to the transistor1101 is equal to the signal current Idata. That is, a gate-sourcevoltage Vgs of the transistor 1101 is necessary for applying the signalcurrent Idata to the transistor 1101. A charge for the gate-sourcevoltage Vgs of the transistor 1101 is accumulated in the capacitor 1104.

It is to be noted that when potentials of the gate terminal and thefirst terminal of the transistor 1101 at this time are denoted by Va andVb respectively, Vgs=(Va−Vb) is satisfied. When a forward thresholdvoltage of the display element 1105 is denoted by V_(ELth),(Vcom−Vb)<V_(ELth) is preferably satisfied, thereby applying no currentto the display element 1105 in the signal writing operation. Therefore,the potential V4 to be inputted to the fourth wire 1109 is desirably setso as to satisfy V3<V4<V5. When V4=Vcom is satisfied, the number ofpower sources necessary for pixels can be reduced. Further, a reversebias can be applied to the display element 1105 in the signal writingoperation.

It is to be noted that even when a reverse bias is applied to thedisplay element 1105, a current does not flow to the display element1105 normally (if flows, it is a slight amount of current). On the otherhand, in the case where the display element 1105 has a short-circuitedportion, a current flows to the short-circuited portion. Then, theshort-circuited portion is insulated, thereby a display defect can beimproved.

Subsequently, description is made with reference to FIG. 12C of thelight emitting operation. A signal inputted to the first wire 1106 isset to be at H level, thereby turning off the first and second switchingtransistors 1102 and 1103. Thus, a current flows as shown in FIG. 12C.At this time, the second switching transistor 1102 is in an off state.Therefore, the capacitor 1104 holds the gate-source voltage Vgsnecessary for applying the signal current Idata to the transistor 1101.Accordingly, a current which is almost equivalent to the signal currentIdata flows to the transistor 1101.

It is to be noted that when potentials of the gate terminal and thefirst terminal of the transistor 1101 at this time are denoted by Va′and Vb′ respectively, Vgs=(Va′−Vb′) is satisfied. This is because Va′ isincreased as Vb′ is increased since the capacitor 1104 holds thegate-source voltage Vgs although Vb′>Vb is satisfied.

It is to be noted that when potentials of L level signal and H levelsignal to be inputted to the first wire 1106 are denoted by V1(L) andV1(H) respectively, the following potentials are preferable. Thresholdvoltages of the first switching transistor 1102 and the second switchingtransistor 1103 are denoted by Vth1 and Vth2 respectively.

As shown in FIG. 12B, even when a potential of the pixel electrode ofthe display element 1105 becomes Vb, the first switching transistor 1102is required to be in an on state. Therefore, V1(L)<(Vb+Vth1) issatisfied. Further, V1(L)<(V4+Vth2) is satisfied in order that thesecond switching transistor 1103 is in an on state. Specifically, forexample, when V4=Vcom is satisfied, V1(L) is preferably a potentiallower than Vcom by 1 to 8 V.

As shown in FIG. 12C, V1(H)>(Vb+Vth1) is satisfied in order that thefirst switching transistor 1102 is turned off. That is, when the signalcurrent is written to another pixel, a potential of the second wire 1107becomes Vb. Therefore, in a pixel which is not selected at this time,the first switching transistor 1102 is required to be in an off state.On the other hand, V1(H)>V4+Vth2 is satisfied in order that the secondswitching transistor 1103 is in an off state. Specifically, for example,when V4=Vcom is satisfied, V1(H) is preferably a potential higher thanVcom by 1 to 8 V.

By employing the pixel structure described in this embodiment mode, apotential of a gate terminal of a transistor in the signal writingoperation can be controlled, thereby preventing a current from flowingto a display element at this time.

By employing the pixel structure shown in FIG. 12, a pixel can be formedof only p-channel transistors, which can simplify manufacturing steps.

Further, by employing the structure of the invention, |Vds|>|Vgs| can besatisfied in the signal writing operation. A change in the Vds can bemade small between in the signal writing operation and in the lightemitting operation. Therefore, even if constant current characteristics(flatness of current) in a saturation region of the transistor 1101 arebad, current values are almost equivalent between in the signal writingoperation and in the light emitting operation. In particular, when anamorphous semiconductor film (such as amorphous silicon) is used for asemiconductor layer of the transistor 1101, constant currentcharacteristics (flatness of current) in a saturation region of thetransistor 1101 may be deteriorated. Thus, when the structure of theinvention is applied in the case where an amorphous semiconductor filmis used for a semiconductor layer of the transistor 1101, a displaydefect can be prevented.

(Embodiment Mode 4)

In this embodiment mode, in particular, description is made of a drivingmethod for reducing a source-drain voltage of a transistor in signalwriting operation and light emitting operation.

Description is made using the pixel of FIG. 1. Since a connectionstructure of the pixel is described in Embodiment Mode 1, descriptionthereof is omitted here.

In this embodiment mode, a potential of the opposed electrode 110 insignal writing operation is higher than that in light emittingoperation. The potential of the opposed electrode 110 at this time isallowable as far as a forward current does not flow to the displayelement 105 in the signal writing operation. The potential may be thesame as or higher than that of the third wire 108.

Moreover, in the signal writing operation, a signal is inputted to thefirst wire 106, thereby turning on the first and second switches 102 and103. Then, a charge for a gate-source voltage necessary for applying thesignal current Idata which flows to the second wire 107 to thetransistor 101 is accumulated in the capacitor 104.

At this time, a potential of the fourth wire 109 to which a gateterminal of the transistor 101 is connected is a predeterminedpotential.

Subsequently, in the light emitting operation, a signal is inputted tothe first wire 106, thereby turning off the first and second switches102 and 103. The potential of the opposed electrode 110 at this time islower than that in the signal writing operation.

The capacitor 104 holds the gate-source voltage necessary for applyingthe signal current Idata to the transistor 101; therefore, a currentwhich is almost equivalent to the signal current Idata flows to thetransistor 101. Then, a current flows to the display element 105.

A potential of a pixel electrode of the display element 105 at this timeis higher than that of the opposed electrode 110. That is, a potentialof a source terminal of the transistor 101 is higher than that of theopposed electrode 110.

Therefore, a potential of the fourth wire 109 for supplying a potentialto be inputted to a gate terminal of the transistor 101 in the signalwriting operation and a potential which is inputted to the opposedelectrode 110 of the display element 105 in the light emitting operationare appropriately set, thereby potential difference of a first terminalof the transistor 101 can be reduced between in the signal writingoperation and in the light emitting operation. Since a predeterminedpotential is inputted to a second terminal of the transistor 101, thepotential of the first terminal of the transistor 101 is controlled,thereby difference of a drain-source voltage of the transistor 101 canbe made small between in the signal writing operation and in the lightemitting operation.

Therefore, even if constant current characteristics (flatness ofcurrent) in a saturation region of the transistor 101 is deteriorated,difference of current values can be made small between in the signalwriting operation and in the light emitting operation. Accordingly,display unevenness can be reduced. In particular, when an amorphoussemiconductor (such as amorphous silicon) is used for a semiconductorlayer of a transistor of a pixel, constant current characteristics(flatness of current) are often deteriorated. Therefore, a drivingmethod of this embodiment mode is applied, thereby preventing a displaydefect.

Further, a potential to be inputted to the fourth wire 109 is set percolumn of pixels in accordance with the amount of signal current,thereby, difference of a drain-source voltage of the transistor 101 canbe made smaller between in the signal writing operation and in the lightemitting operation. Accordingly, the transistor 101 may be operated in alinear region.

(Embodiment Mode 5)

In this embodiment mode, by using a timing chart shown in FIG. 40,description is made of one mode of a driving method of a display deviceto which a pixel of the invention can be applied. Further, a pixelstructure of the invention, to which the driving method can be applied,is described.

Horizontal direction indicates passage of time and longitudinaldirection indicates the number of scan rows of scan lines.

When images are displayed, writing operation and light emittingoperation are repeated. A period in which writing operation and lightemitting operation for one screen (one frame) are performed is referredto as one frame period. Although there is no particular limitation on aprocess of signals for one frame, it is preferable that the number ofone frame periods be at least about 60 times per second so as not tomake a viewer notice flickers.

In a display device of this embodiment mode, a video signal is writtento a pixel in accordance with a gray scale of each pixel. In otherwords, an analog signal is written to a pixel. The video signal is asignal current.

In a light emitting period, a gray scale is expressed by holding thevideo signal. Here, a display device including a pixel of thisembodiment mode erases a signal written to a pixel by erasing operation.Thus, an erasing period is provided until a next frame period. That is,black display is inserted, thereby persistence of vision can be hardlyseen. Accordingly, characteristics of a moving image can be improved.

Description is made of a pixel structure to which a driving method ofthis embodiment mode can be applied. A pixel of this embodiment mode isallowable as far as it has a means of forcibly making a pixel emit nolight by scanning. As such a means, in the case of the pixel shown inFIG. 1, a path of a current from the third wire 108 to the opposedelectrode 110 of the display element 105 through the transistor 101 ispreferably made non-conductive.

There are roughly two methods of making the path of a currentno-conductive. As one method, another switch is provided in the path ofa current from the third wire 108 to the opposed electrode 110 of thedisplay element 105 through the transistor 101. Then, the switch isturned off by scanning a pixel per row, thereby the path of a current ismade non-conductive.

An example of such a structure is shown in FIG. 42. Note that commonportions to those in FIG. 1 are denoted by the same reference numeralsand description thereof is omitted.

In a structure of FIG. 42, a third switch 4201 is connected between asecond terminal of a first transistor 101 and a third wire 108, based onthe structure of FIG. 1. The third switch 4201 is controlled to beturned on or off by a signal to be inputted to a fifth wire 4202. Notethat a portion where the switch is provided is not limited to thestructure of FIG. 42. When a connecting point of the first terminal ofthe transistor 101 and a pixel electrode of a display element 105 is anode 4203, the switch may be connected between the node 4203 and thefirst terminal of the transistor 101 or the pixel electrode of thedisplay element 105.

As the other method, the transistor 101 is forcibly turned off byscanning a pixel per row. Therefore, the pixel is required to have ameans of discharging a charge accumulated in a capacitor 104 or a meansof inputting a potential to a gate terminal of the transistor 101.

First, FIG. 38 shows one example of a pixel having a means ofdischarging a charge accumulated in a capacitor 104. Note that commonportions to those in FIG. 1 are denoted by the same reference numeralsand description thereof is omitted. In FIG. 38, the capacitor 104 and athird switch 3801 are connected in parallel. The third switch 3801 iscontrolled to be turned on or off by a signal to be inputted to a fifthwire 3802. That is, when the third switch 3801 is turned on, the gateterminal and the first terminal of the transistor 101 areshort-circuited. Thus, a gate-source voltage of the transistor 101,which is held in the capacitor 104, can be set to 0 V. Accordingly, thetransistor 101 can be turned off.

It is to be noted that by employing a structure of FIG. 5 or FIG. 49, acharge accumulated in the capacitor 104 can be discharged by scanning apixel per row. In this case, a second switch 103 is turned on by asignal to be supplied to a fifth wire 501. The second switch 103 isturned on while the first switch 102 is in an off state, thereby acharge accumulated in the capacitor 104 is discharged through thetransistor 101. Accordingly, the transistor 101 can be turned off.

Further, FIG. 39 shows one example of a pixel having a means ofinputting a potential to the gate terminal of the transistor 101. Notethat common portions to those in FIG. 1 are denoted by the samereference numerals and description thereof is omitted. In FIG. 39, arectifying element 3901 is connected between the gate terminal of thefirst transistor 101 and a fifth wire 3902. The rectifying element 3901is connected so that direction of a current flowing from the gateterminal of the transistor 101 to the fifth wire 3902 is a forwardcurrent. Only in the case where the transistor 101 is forcibly turnedoff, L level signal is inputted to the fifth wire 3902, and in the othercases, H level signal is inputted to the fifth wire 3902. Accordingly,when the fifth wire 3902 is at H level, a current does not flow to therectifying element 3901 whereas when the fifth wire 3902 is at L level,a current flows from the transistor 101 to the fifth wire 3902.Therefore, a potential of the gate terminal of the transistor 101 ishigher than that of the fifth wire 3902 at L level by a forwardthreshold voltage of the rectifying element 3901. At this time, a chargeis accumulated also in a second electrode of the capacitor 104 throughthe transistor 101. Then, a potential of the first terminal of thetransistor 101 also becomes high. Thus, the transistor 101 can beforcibly turned off.

As another example of a pixel having a means of inputting a potential tothe gate terminal of the transistor 101, a pixel structure of FIG. 5 maybe employed. In this case, the second switch 103 is turned on byinputting a signal to the fifth wire 501, thereby a charge isaccumulated in the second electrode of the capacitor 104 through thetransistor 101. Accordingly, the transistor 101 is turned off.

Further, FIG. 41 shows another example of a pixel having a means ofinputting a potential to the gate terminal of the transistor 101. Notethat common portions to those in FIG. 1 are denoted by the samereference numerals and description thereof is omitted. In FIG. 41, athird switch 4101 is connected between the gate terminal of thetransistor 101 and the opposed electrode 110 of the display element 105.The third switch 4101 and the opposed electrode 110 of the displayelement 105 are connected through a wire 4103. Note that the thirdswitch 4101 is controlled to be turned on or off by inputting a signalto a fifth wire 4102. When the third switch 4101 is turned on byinputting a signal to the fifth wire 4102, a charge of the capacitor 104is discharged through the transistor 101. Accordingly, the transistor101 is turned off.

Note that a cross sectional structure of a display panel having pixelsshown in FIG. 41 is described with reference to FIG. 43.

A base film 4302 is provided over a substrate 4301. The substrate 4301can be formed of an insulating substrate such as a glass substrate, aquartz substrate, a plastic substrate, or a ceramic substrate, or of ametal substrate, a semiconductor substrate, or the like. The base film4302 can be formed by CVD or sputtering. For example, a silicon oxidefilm, a silicon nitride film, a silicon oxynitride film, or the likeformed by CVD using SiH₄, N₂O, and NH₃ as a source material. Moreover, astacked layer of them may be used as well. It is to be noted that thebase film 4302 is provided to prevent impurities from dispersing fromthe substrate 4301 into the semiconductor layer. When the substrate 4301is formed of a glass substrate or a quartz substrate, the base film 4302is not required to be provided.

Island-shaped semiconductor layers are formed over the base film 4302.In each of the semiconductor layers, a channel forming region 4303 wherean n-channel is formed, an impurity region 4304 which functions as asource region or a drain region, and a low concentration impurity region(LDD region) 4305 are formed. A gate electrode 4307 is formed over thechannel forming region 4303 with a gate insulating film 4306 interposedtherebetween. As the gate insulating film 4306, a silicon oxide film, asilicon nitride film, a silicon oxynitride film, or the like formed byCVD or sputtering can be used. Further, an aluminum (Al) film, a copper(Cu) film, a thin film containing aluminum or copper as a maincomponent, a chromium (Cr) film, a tantalum (Ta) film, a tantalumnitride (TaN) film, a titanium (Ti) film, a tungsten (W) film, amolybdenum (Mo) film, or the like can be used as the gate electrode4307.

Sidewalls 4322 are formed on the sides of the gate electrode 4307. Afterforming a silicon compound, for example, a silicon oxide film, a siliconnitride film, or a silicon oxynitride film is formed so as to cover thegate electrode 4307, etch-back treatment is applied to form thesidewalls 4322.

The LDD regions 4305 are formed under the sidewalls 4322. That is, theLDD regions 4305 are formed in a self-aligned manner. Note that thesidewalls 4322 are not necessarily provided since they are provided toform the LDD regions 4305 in a self-aligned manner.

A first interlayer insulating film is formed over the gate electrode4307, the sidewalls 4322, and the gate insulating film 4306. The firstinterlayer insulating film includes an inorganic insulating film 4318 asa lower layer and a resin film 4308 as an upper layer. As the inorganicinsulating film 4318, a silicon nitride film, a silicon oxide film, asilicon oxynitride film, or a film formed by stacking these layers canbe used. As the resin film 4308, polyimide, polyamide, acrylic,polyimide amide, epoxy, or the like can be used.

A first electrode 4309, a second electrode 4324, a third electrode 4320,and a fourth electrode 4321 are formed over the first interlayerinsulating film. The first electrode 4309, the second electrode 4324,and the fourth electrode 4321 are electrically connected to the impurityregions 4304 through contact holes. Further, the third electrode 4320 iselectrically connected to the gate electrode 4307 through a contacthole. The third electrode 4320 and the fourth electrode 4321 areelectrically connected to each other. A titanium (Ti) film, an aluminum(Al) film, a copper (Cu) film, an aluminum film containing Ti, or thelike can be used as the first electrode 4309 and the second electrode4324. It is to be noted that in the case of providing a wire such as asignal line in the same layer as the first electrode 4309, the secondelectrode 4324, the third electrode 4320, and the fourth electrode 4321,copper which has low resistance is preferably used.

A second interlayer insulating film 4310 is formed over the firstelectrode 4309, the second electrode 4324, the third electrode 4320, thefourth electrode 4321, and the first interlayer insulating film. As thesecond interlayer insulating film 4310, an inorganic insulating film, aresin film, or a film formed by stacking these layers can be used. As aninorganic insulating film, a silicon nitride film, a silicon oxide film,a silicon oxynitride film, or a film formed by stacking these layers canbe used. As a resin film, polyimide, polyamide, acrylic, polyimideamide, epoxy, or the like can be used.

A pixel electrode 4311 and a wire 4319 are formed over the secondinterlayer insulating film 4310. The pixel electrode 4311 and the wire4319 are formed of the same material. That is, they are formed in thesame layer at the same time. As a material used for the pixel electrode4311 and the wire 4319, a material having a high work function ispreferably used. For example, a single layer of a titanium nitride (TiN)film, a chromium (Cr) film, a tungsten (W) film, a zinc (Zn) film, aplatinum (Pt) film, or the like, a stacked layer of a titanium nitridefilm and a film containing aluminum as a main component, a stacked layerof three layers of a titanium nitride film, a film containing aluminumas a main component, and a titanium nitride film can be used. With astacked layer structure, the resistance as a wire is low, a preferableohmic contact can be obtained, and further a function as an anode can beobtained. By using a metal film which reflects light, an anode whichdoes not transmit light can be formed.

An insulator 4312 is formed so as to cover end portions of the pixelelectrode 4311 and the wire 4319. As the insulator 4312, for example, apositive type photosensitive acrylic resin film can be used.

A layer 4313 containing an organic compound is formed over the pixelelectrode 4311, and the layer 4313 containing an organic compoundpartially overlaps the insulator 4312. Note that the layer 4313containing an organic compound is not formed over the wire 4319.

An opposed electrode 4314 is provided over the layer 4313 containing anorganic compound, the insulator 4312, and the wire 4319. As a materialused for the opposed electrode 4314, a material having a low workfunction is preferably used. For example, a metal thin film of aluminum(Al), silver (Ag), lithium (Li), calcium (Ca), an alloy of these, MgAg,MgIn, AlLi, CaF₂, Ca₃N₂ or the like can be used. By using a metal thinfilm in this manner, a cathode which can transmit light can be formed.

A region where the layer 4313 containing an organic compound isinterposed between the opposed electrode 4314 and the pixel electrode4311 corresponds to a light emitting element 4316.

In a region where the layer 4313 containing an organic compound isisolated by the insulator 4312, a joint portion 4317 is formed so thatthe opposed electrode 4314 and the wire 4319 contact each other.Therefore, the wire 4319 functions as an auxiliary electrode of theopposed electrode 4314, thereby the lower resistance of the opposedelectrode 4314 can be realized. Accordingly, a film thickness of theopposed electrode 4314 can be reduced, which leads to an increase in thelight transmittance. Therefore, higher luminance can be obtained in atop emission structure where the light from the light emitting element4316 is extracted from a top surface.

A stacked layer of a metal thin film and a light-transmissive conductivefilm (such as ITO (indium tin oxide) film, indium zinc oxide (IZO) film,or zinc oxide (ZnO) film) may be used in order to realize the lowerresistance of the opposed electrode 4314. In this manner, a cathodewhich can transmit light can be formed by using a metal thin film and alight-transmissive conductive film which transmits light as well.

That is, a transistor 4315 corresponds to the transistor 101 in thepixel of FIG. 41 and a transistor 4323 fulfills the function of thethird transistor 4101 in the pixel of FIG. 41. Further, the opposedelectrode 4314 corresponds to the opposed electrode 110 of the displayelement 105 in the pixel of FIG. 41. In addition, the wire 4319corresponds to the wire 4103 in the pixel of FIG. 41.

Further, a display panel having the structure shown in FIG. 43 can alsobe applied in the case of having another pixel structure. For example,the transistor 4315 corresponds to the transistor 101 of FIG. 48 or 49,and the transistor 4323 fulfills the function of the second switch 103of FIG. 48 or 49. Note that the electrode 4324 may correspond to thefourth wire 109 of FIG. 48 or 49, and the wire 4319 may correspond tothe fifth wire 4801 of FIG. 48 or the sixth wire 4901 of FIG. 49.Alternatively, the wire 4319 may fulfill the function of the fourth wire109 and the fifth wire 4801 of FIG. 48 or of the fourth wire 109 and thesixth wire 4901 of FIG. 49.

In the display panel having the structure shown in FIG. 43, the film ofthe opposed electrode 4314 can be formed thin, thereby the light can beemitted from a top surface with favorable transmittance. Therefore, theluminance from the top surface can be enhanced. Further, by connectingthe opposed electrode 4314 and the wire 4319, the lower resistance ofthe opposed electrode 4314 and the wire 4319 can be realized. Therefore,power consumption can be reduced.

Further, the transistor 101 can be forcibly turned off by in a displaydevice having the pixel structure of FIG. 2. A driving method in thiscase is described below.

One horizontal period is divided into two periods as shown in FIG. 44.Here, description is made assuming that the former half is writing timeand the latter half is erasing time. In the divided horizontal period,each scan line is selected, and at that time, a corresponding signal isinputted to a signal line. For example, an i-th row is selected in theformer half of a certain horizontal period and a j-th row is selected inthe latter half. Then, operation can be performed as if two rows areselected at the same time in one horizontal period. In other words, thevideo signals are written to pixels from the signal line in the writingtime Tb1 to Tb4 using writing time that is the former half of eachhorizontal period. Then, a pixel is not selected in erasing time that isthe latter half of the one horizontal period at this time. In addition,an erasing signal is inputted to a pixel from the signal line in erasingtime Te using erasing time that is the latter half of another horizontalperiod. In writing time that is the former half of one horizontal periodat this time, a pixel is not selected. Thus, a display device having ahigh aperture ratio can be provided and the yield can be improved.

FIG. 45 shows an example of a display device including such a pixel. Thedisplay device has a signal line driver circuit 4501, a first scan linedriver circuit 4502, a second scan line driver circuit 4505, and a pixelportion 4503 in which pixels 4504 are arranged in matrix correspondingto scan lines G1 to Gm and signal lines S1 to Sn. The first scan linedriver circuit 4502 includes a pulse output circuit 4506 and switches4508 which are connected between each of the scan lines G1 to Gm and thepulse output circuit 4506. The second scan line driver circuit 4505includes a pulse output circuit 4507 and switches 4509 which areconnected between each of the scan lines G1 to Gm and the pulse outputcircuit 4507.

It is to be noted that a scan line Gi (one of the scan lines G1 to Gm)corresponds to the first wire 106 of FIG. 2, and a signal line Sj (oneof the signal lines S1 to Sn) corresponds to the second wire 107 of FIG.2

A clock signal (G_CLK), an inverted clock signal (G_CLKB), a start pulsesignal (G_SP), a control signal (WE), and the like are inputted to thefirst scan line driver circuit 4502. In accordance with these signals,signals selecting pixels are outputted to a first scan line Gi (one ofthe first scan lines G1 to Gm) of a pixel row to be selected. Note thatthe signals at this time are pulses outputted in the former half of onehorizontal period as shown in a timing chart of FIG. 37. The switches4508 are controlled to be turned on or off by the control signal (WE),thereby the pulse output circuit 4506 and the scan lines G1 to Gm can beelectrically connected or disconnected.

A clock signal (R_CLK), an inverted clock signal (R_CLKB), a start pulsesignal (R_SP), a control signal (WE′), and the like are inputted to thesecond scan line driver circuit 4505. In accordance with these signals,signals are outputted to a second scan line Ri (one of the second scanlines R1 to Rm) of a pixel row to be selected. Note that the signals atthis time are pulses outputted in the latter half of one horizontalperiod as shown in the timing chart of FIG. 37. The switches 4509 arecontrolled to be turned on or off by the control signal (WE′), therebythe pulse output circuit 4507 and the scan lines G1 to Gm can beelectrically connected or disconnected. Note that when one of theswitches 4508 and the switches 4509 are electrically connected, theother is electrically disconnected.

A clock signal (S_CLK), an inverted clock signal (S_CLKB), a start pulsesignal (S_SP), a video signal (Digital Video Data), a control signal(WE), and the like are inputted to the signal line driver circuit 4501.In accordance with these signals, a video signal corresponding to pixelsof each row is outputted to each of the signal lines S1 to Sn.

Therefore, the video signal inputted to the signal lines S1 to Sn iswritten to the pixel 4504 of each column in the row selected by a signalinputted to the scan line Gi (one of the scan lines G1 to Gm) from thefirst scan line driver circuit 4502. Then, each pixel row is selectedthrough each of the scan lines G1 to Gm, thereby video signalscorresponding to each of the pixels 4504 are inputted to all the pixels4504. Each of the pixels 4504 holds the data of the written video signalfor a certain period. Then, each of the pixels 4504 can keep a lightemitting state or a non-light emitting state by holding the data of thevideo signal for a certain period.

Further, a signal (also referred to as erasing signal) for making apixel emit no light is written from the signal lines S1 to Sn to thepixel 4504 of each column in the row selected by a signal inputted tothe scan line Gi (one of the scan lines G1 to Gm) from the second scanline driver circuit 4505. Then, each pixel row is selected by each ofthe scan lines G1 to Gm, thereby setting a non-light emitting period.For example, when the pixel in an i-th row is selected by the signalinputted from the second scan line driver circuit 4505 to the scan lineGi, the potentials of the signal lines S1 to Sn are the same as that ofthe fourth wire 109 in the pixel of FIG. 2. Note that the signal linesS1 to Sn may be in a floating state at this time.

Therefore, by using a display device of the invention, in the case offocusing on a certain pixel row, when a signal inputted to the certainpixel row is the same as that to be inputted, the signal can beprevented from being inputted to the pixel row, which leads to reductionin the number of times charging and discharging the scan line or thesignal line are performed. As a result, power consumption can belowered.

(Embodiment Mode 6)

In this embodiment mode, description is made with reference to FIGS. 22Aand 22B of the structures of a display panel having the pixel structuredescribed in Embodiment Modes 1 to 3.

It is to be noted that FIG. 22A is a top plan view of the display paneland FIG. 22B is a cross sectional diagram along a line A-A′ of FIG. 22A.The display panel includes a signal line driver circuit 2201, a pixelportion 2202, a first scan line driver circuit 2203, and a second scanline driver circuit 2206, which are shown by dotted lines. Further, asealing substrate 2204 and a sealing material 2205 are provided. Aportion surrounded by the sealing material 2205 is a space 2207.

It is to be noted that a wire 2208 is a wire for transmitting a signalinputted to the first scan line driver circuit 2203, the second scanline driver circuit 2206, and the signal line driver circuit 2201 andreceives a video signal, a clock signal, a start signal, and the likefrom an FPC (Flexible Printed Circuit) 2209 functioning as an externalinput terminal. An IC chip (semiconductor chip including memory circuit,buffer circuit, and the like) 2219 is mounted over a connecting portionof the FPC 2209 and the display panel by COG (Chip On Glass) or thelike. It is to be noted that only the FPC 2209 is shown here; however, aprinted wiring board (PWB) may be attached to the FPC 2209. The displaydevice in this specification includes not only a main body of thedisplay panel but one with an FPC or a PWB attached thereto and one onwhich an IC chip or the like is mounted.

Next, description is made with reference to FIG. 22B of across-sectional structure. The pixel portion 2202 and peripheral drivercircuits (the first scan line driver circuit 2203, the second scan linedriver circuit 2206, and the signal line driver circuit 2201) are formedover a substrate 2210. Here, the signal line driver circuit 2201 and thepixel portion 2202 are shown.

It is to be noted that the signal line driver circuit 2201 is formed ofunipolar transistors such as n-channel TFT's 2220 and 2221. As for apixel structure, a pixel can be formed of a unipolar transistor byapplying the pixel structure of FIG. 2, 13, 14, or 15. Accordingly, theperipheral driver circuits are formed of n-channel transistors, therebya unipolar display panel can be manufactured. Needless to say, a CMOScircuit may be formed of a p-channel transistor as well as a unipolartransistor. Further, in this embodiment mode, a display panel in whichthe peripheral driver circuits are formed over the same substrate isshown; however, the invention is not limited to this. All or some of theperipheral driver circuits may be formed into an IC chip or the like andmounted by COG or the like. In this case, the driver circuit is notrequired to be unipolar and can be formed in combination with ap-channel transistor.

Further, the pixel portion 2202 includes TFTs 2211 and 2212. It is to benoted that a source electrode of the TFT 2212 is connected to a firstelectrode (pixel electrode) 2213. An insulator 2214 is formed so as tocover end portions of the first electrode 2213. Here, a positivephotosensitive acrylic resin film is used for the insulator 2214.

In order to obtain favorable coverage, the insulator 2214 is formed sothat a curved surface having a curvature is formed at a top end portionor a bottom end portion of the insulator 2214. For example, in the caseof using a positive photosensitive acrylic as a material for theinsulator 2214, it is preferable that only the top end portion of theinsulator 2214 have a curved surface having a curvature radius (0.2 to 3μm). Moreover, either a negative photosensitive acrylic which becomesinsoluble in etchant by light or a positive photosensitive acrylic whichbecomes soluble in etchant by light can be used as the insulator 2214.

A layer 2216 containing an organic compound and a second electrode(opposed electrode) 2217 are formed over the first electrode 2213. Here,it is preferable to use a material having a high work function as amaterial used for the first electrode 2213 which functions as an anode.For example, a single layer of an ITO (indium tin oxide) film, an indiumzinc oxide (IZO) film, a titanium nitride film, a chromium film, atungsten film, a Zn film, a Pt film, or the like, a stacked layer of atitanium nitride film and a film containing aluminum as a maincomponent, a three-layer structure of a titanium nitride film, a filmcontaining aluminum as a main component, and a titanium nitride film, orthe like can be used. It is to be noted that with a stacked layerstructure, resistance as a wire is low, favorable ohmic contact can beobtained, and a function as an anode can be obtained.

The layer 2216 containing an organic compound is formed by vapordeposition using a deposition mask, or ink-jet. A metal complexbelonging to group 4 of the periodic table of elements is used for apart of the layer 2216 containing an organic compound. Besides, a lowmolecular material or a high molecular material may be used incombination as well. Further, as a material used for the layer 2216containing an organic compound, a single layer or a stacked layer of anorganic compound is often used; however, in this embodiment mode, aninorganic compound may be used in a part of a film formed of an organiccompound. Moreover, a known triplet material can also be used.

Further, as a material used for the second electrode 2217 whichfunctions as a cathode and is formed over the layer 2216 containing anorganic compound, a material having a low work function (Al, Ag, Li, Ca,or an alloy thereof such as MgAg, MgIn, AlLi, CaF₂, or Ca₃N₂) may beused. In the case where light generated from the layer 2216 containingan organic compound is transmitted through the second electrode 2217, astacked layer of a metal thin film with a thinner thickness and alight-transmissive conductive film (TTO (indium tin oxide) film), indiumoxide zinc oxide alloy (In₂O₃—ZnO), zinc oxide (ZnO), or the like) ispreferably used.

Further, by attaching the sealing substrate 2204 to the substrate 2210with the sealing material 2205, a light emitting element 2218 isprovided in the space 2207 surrounded by the substrate 2210, the sealingsubstrate 2204, and the sealing material 2205. It is to be noted thatthe space 2207 may be filled with the sealing material 2205 as well asan inert gas (nitrogen, argon, or the like).

It is to be noted that an epoxy-based resin is preferably used for thesealing material 2205. Further, it is preferable that these materialsshould not transmit moisture or oxygen as much as possible. As amaterial for the sealing substrate 2204, a glass substrate, a quartzsubstrate, a plastic substrate formed of FRP (Fiberglass-ReinforcedPlastics), PVF (polyvinylfluoride), myler, polyester, acrylic, or thelike can be used.

As described above, a display panel having a pixel structure of theinvention can be obtained. Note that the structure described above isone example, and a structure of a display panel is not limited to this.

As shown in FIGS. 22A and 22B, the cost of the display device can bereduced by forming the signal line driver circuit 2201, the pixelportion 2202, the first scan line driver circuit 2203, and the secondscan line driver circuit 2206 over the same substrate. Further, in thiscase, unipolar transistors are used for the signal line driver circuit2201, the pixel portion 2202, the first scan line driver circuit 2203,and the second scan line driver circuit 2206, thereby manufacturingsteps can be simplified. As a result, further cost reduction can beachieved.

It is to be noted that the structure of the display panel is not limitedto the structure shown in FIG. 22A where the signal line driver circuit2201, the pixel portion 2202, the first scan line driver circuit 2203,and the second scan line driver circuit 2206 are formed over the samesubstrate, and a signal line driver circuit 2801 shown in FIG. 28Acorresponding to the signal line driver circuit 2201 may be formed intoan IC chip and mounted on the display panel by COG, or the like. It isto be noted that a substrate 2800, a pixel portion 2802, a first scanline driver circuit 2803, a second scan line driver circuit 2804, an FPC2805, IC chips 2806 and 2807, a sealing substrate 2808, and a sealingmaterial 2809 in FIG. 28A correspond to the substrate 2210, the pixelportion 2202, the first scan line driver circuit 2203, the second scanline driver circuit 2206, the FPC 2209, the IC chips 2219 and 2222, thesealing substrate 2204, and the sealing material 2205 in FIG. 22A,respectively.

That is, only the signal line driver circuit which is required tooperate at high speed is formed into an IC chip using a CMOS or thelike, thereby lower power consumption is achieved. Further, by formingthe IC chip into a semiconductor chip formed of a silicon wafer or thelike, higher-speed operation and lower power consumption can berealized.

By forming the second scan line driver circuit 2803 and the first scanline driver circuit 2804 over the same substrate as the pixel portion2802, cost reduction can be achieved. Further, unipolar transistors areused for the second scan line driver circuit 2803, the first scan linedriver circuit 2804, and the pixel portion 2802, thereby further costreduction can be achieved. As for a pixel structure of the pixel portion2802, the structures described in Embodiment Modes 1 to 4 can beapplied.

In this manner, cost reduction of a high definition display device canbe realized. Further, by mounting an IC chip including a functionalcircuit (memory or buffer) at a connecting portion of the FPC 2805 andthe substrate 2800, a substrate area can be efficiently utilized.

Moreover, a signal line driver circuit 2811, a first scan line drivercircuit 2814, and a second scan line driver circuit 2813 shown in FIG.28B corresponding to the signal line driver circuit 2201, the first scanline driver circuit 2203, and the second scan line driver circuit 2206shown in FIG. 22A may be formed into an IC chip and mounted on a displaypanel by COG or the like. In this case, lower power consumption of ahigh definition display device can be realized. Therefore, in order toobtain a display device with less power consumption, it is preferable touse polysilicon for a semiconductor layer of a transistor used in thepixel portion. It is to be noted that a substrate 2810, a pixel portion2812, an FPC 2815, IC chips 2816 and 2817, a sealing substrate 2818, anda sealing material 2822 in FIG. 28B correspond to the substrate 2210,the pixel portion 2202, the FPC 2209, the IC chips 2219 and 2222, thesealing substrate 2204, and the sealing material 2205 in FIG. 22A,respectively.

Further, by using amorphous silicon for a semiconductor layer of atransistor of the pixel portion 2812, further cost reduction can beachieved. Moreover, a large display panel can be manufactured.

Further, the second scan line driver circuit, the first scan line drivercircuit, and the signal line driver circuit are not necessarily providedin a row direction and a column direction of the pixels. For example, asshown in FIG. 29A, a peripheral driver circuit 2901 formed in an IC chipmay have functions of the first scan line driver circuit 2814, thesecond scan line driver circuit 2813, and the signal line driver circuit2811 shown in FIG. 28B. It is to be noted that a substrate 2900, a pixelportion 2902, an FPC 2904, IC chips 2905 and 2906, a sealing substrate2907, and a sealing material 2908 in FIG. 29A correspond to thesubstrate 2210, the pixel portion 2202, the FPC 2209, the IC chips 2219and 2222, the sealing substrate 2204, and the sealing material 2205 inFIG. 22A, respectively.

FIG. 29B shows a schematic diagram showing connections of wires of thedisplay device shown in FIG. 29A. A substrate 2910, a peripheral drivercircuit 2911, a pixel portion 2912, and FPCs 2913 and 2914 are provided.Signals and a power source potential are externally inputted from theFPC 2913 to the peripheral driver circuit 2911. An output from theperipheral driver circuit 2911 is inputted to wires in the row directionand wires in the column direction, which are connected to the pixels inthe pixel portion 2912.

Further, FIGS. 23A and 23B show examples of a light emitting elementwhich can be applied to the light emitting element 2218. That is,description is made with reference to FIGS. 23A and 23B of structures ofa light emitting element which can be applied to the pixels described inEmbodiment Modes 1 to 4.

In a light emitting element shown in FIG. 23A, an anode 2302, a holeinjecting layer 2303 formed of a hole injecting material, a holetransporting layer 2304 formed of a hole transporting material, a lightemitting layer 2305, an electron transporting layer 2306 formed of anelectron transporting material, an electron injecting layer 2307 formedof an electron injecting material, and a cathode 2308 are stacked over asubstrate 2301 in this order. Here, the light emitting layer 2305 may beformed of only one kind of light emitting material; however, it may alsobe formed of two or more kinds of materials. The structure of theelement of the invention is not limited to this.

In addition to the stacked layer structure shown in FIG. 23A where eachfunctional layer is stacked, there are wide variations such as anelement formed of a high molecular compound, a high efficiency elementwhich utilizes a triplet light emitting material which emits light froma triplet excitation state in a light emitting layer. It is alsopossible to apply to a white light emitting element which can beobtained by dividing a light emitting region into two regions bycontrolling a recombination region of carriers using a hole blockinglayer, and the like.

The element of the invention shown in FIG. 23A can be formed bysequentially depositing a hole injecting material, a hole transportingmaterial, and a light emitting material over the substrate 2301 havingthe anode 2302 (ITO). Next, an electron transporting material and anelectron injecting material are deposited, and finally the cathode 2808is deposited.

Materials suitable for the hole injecting material, the holetransporting material, the electron transporting material, the electroninjecting material, and the light emitting material are as follows.

As the hole injecting material, an organic compound such as aporphyrin-based compound, a phthalocyanine (hereinafter referred to as“H₂Pc”), copper phthalocyanine (hereinafter referred to as “CuPc”), orthe like is effective. Further, a material that has a smaller value ofan ionization potential than that of the hole transporting material tobe used and has a hole transporting function can also be used as thehole injecting material. There is also a material obtained by chemicallydoping a conductive high molecular compound, which includes polyanilineand polyethylene dioxythiophene (hereinafter referred to as “PEDOT”)doped with polystyrene sulfonate (hereinafter referred to as “PSS”).Also, a high molecular compound of an insulator is effective in terms ofplanarization of an anode, and polyimide (hereinafter referred to as“PI”) is often used. Further, an inorganic compound is also used, whichincludes an extra-thin film of aluminum oxide (hereinafter referred toas “alumina”) in addition to a thin film of a metal such as gold orplatinum.

It is an aromatic amine-based (that is, one having a bond of benzenering-nitrogen) compound that is most widely used as the holetransporting material. A material that is widely used includes4,4′-bis(diphenylamino)-biphenyl (hereinafter referred to as “TAD”),derivatives thereof such as4,4′-bis[N-(3-methylphenyl)-N-phenyl-amino]-biphenyl (hereinafterreferred to as “TPD”), 4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl(hereinafter referred to as “α-NPD”), and star burst aromatic aminecompounds such as 4,4′,4″-tris(N, N-diphenyl-amino)-triphenylamine(hereinafter referred to as “TDATA”) and4,4′,4″-tris[N-(3-methylphenyl)-N-phenyl-amino]-triphenylamine(hereinafter referred to as “MTDATA”).

As the electron transporting material, a metal complex is often used,which includes a metal complex having a quinoline skeleton or abenzoquinoline skeleton such as Alq, BAlq,tris(4-methyl-8-quinolinolato)aluminum (hereinafter referred to as“Almq”), or bis(10-hydroxybenzo[h]-quinolinato)beryllium (hereinafterreferred to as “BeBq”), and in addition, a metal complex having anoxazole-based or a thiazole-based ligand such asbis[2-(2-hydroxyphenyl)-benzoxazolato]zinc (hereinafter referred to as“Zn(BOX)₂”) or bis[2-(2-hydroxyphenyl)-benzothiazolato]zinc (hereinafterreferred to as “Zn(BTZ)₂”). Further, in addition to the metal complexes,oxadiazole derivatives such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (hereinafterreferred to as “PBD”) and OXD-7, triazole derivatives such as TAZ and3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-2,3,4-triazole(hereinafter referred to as “p-EtTAZ”), and phenanthroline derivativessuch as bathophenanthroline (hereinafter referred to as “BPhen”) and BCPhave an electron transporting property.

As the electron injecting material, the above-mentioned electrontransporting materials can be used. In addition, an extra-thin film ofan insulator, for example, metal halide such as calcium fluoride,lithium fluoride, or cesium fluoride, alkali metal oxide such as lithiumoxide, or the like is often used. Further, an alkali metal complex suchas lithium acetyl acetonate (hereinafter referred to as “Li(acac)”) or8-quinolinolato-lithium (hereinafter referred to as “Liq”) is alsoeffective.

As the light emitting material, in addition to the above-mentioned metalcomplexes such as Alq, Almq, BeBq, BAlq, Zn(BOX)₂, and Zn(BTZ)₂, variousfluorescent pigments are effective. The fluorescent pigments include4,4′-bis(2,2-diphenyl-vinyl)-biphenyl, which is blue, and4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran, whichis red-orange, and the like. Also, a triplet light emitting material isavailable, which mainly includes a complex with platinum or iridium as acentral metal. As the triplet light emitting material,tris(2-phenylpyridine)iridium,bis(2-(4′-tryl)pyridinato-N,C^(2′))acetylacetonato iridium (hereinafterreferred to as “acacIr(tpy)₂”),2,3,7,8,12,13,17,18-octaethyl-21H,23Hporphyrin-platinum, and the likeare known.

By using the materials each having a function as described above incombination, a highly reliable light emitting element can be formed.

In the case of the pixel of FIG. 11, which is described in EmbodimentMode 3, a light emitting element in which layers are formed in a reverseorder to that of FIG. 23A can be used as shown in FIG. 23B. That is, acathode 2318, an electron injecting layer 2317 formed of an electroninjecting material, an electron transporting layer 2316 formed of anelectron transporting material, a light emitting layer 2315, a holetransporting layer 2314 formed of a hole transporting material, a holeinjecting layer 2313 formed of a hole injecting material, and an anode2312 are stacked in this order over a substrate 2311.

In addition, in order to extract light emission of a light emittingelement, at least one of an anode and a cathode is required to transmitlight. A TFT and a light emitting element are formed over a substrate;and there are light emitting elements having a top emission structurewhere light emission is taken out through a surface opposite to thesubstrate, having a bottom emission structure where light emission istaken out through a surface on the substrate side, and having a dualemission structure where light emission is taken out through a surfaceopposite to the substrate and a surface on the substrate siderespectively. The pixel structure of the invention can be applied to thelight emitting element having any emission structure.

Description is made with reference to FIG. 24A of a light emittingelement with a top emission structure.

A driving TFT 2401 is formed over a substrate 2400 and a first electrode2402 is formed in contact with a source electrode of the driving TFT2401, over which a layer 2403 containing an organic compound and asecond electrode 2404 are formed.

Further, the first electrode 2402 is an anode of a light emittingelement. The second electrode 2404 is a cathode of the light emittingelement. That is, a region where the layer 2403 containing an organiccompound is interposed between the first electrode 2402 and the secondelectrode 2404 corresponds to the light emitting element.

Further, as a material used for the first electrode 2402 which functionsas an anode, a material having a high work function is preferably used.For example, a single layer of a titanium nitride film, a chromium film,a tungsten film, a Zn film, a Pt film, or the like, a stacked layer of atitanium nitride film and a film containing aluminum as a maincomponent, a stacked layer of three layers of a titanium nitride film, afilm containing aluminum as a main component, and a titanium nitridefilm, or the like can be used. With a stacked layer structure, theresistance as a wire is low, a preferable ohmic contact can be obtained,and further a function as an anode can be obtained. By using a metalfilm which reflects light, an anode which does not transmit light can beformed.

As a material used for the second electrode 2404 which functions as acathode, a stacked layer of a metal thin film formed of a materialhaving a low work function (Al, Ag, Li, Ca, or an alloy thereof such asMgAg, MgIn, AlLi, CaF₂, or Ca₃N₂) and a light-transmissive conductivefilm (of ITO (indium tin oxide), indium zinc oxide (IZO), zinc oxide(ZnO), or the like) is preferably used. By using a metal thin film and alight-transmissive conductive film in this manner, a cathode which cantransmit light can be formed.

In this manner, light from the light emitting element can be extractedto the top surface as shown by an arrow in FIG. 24A. That is, in thecase of applying to the display panel shown in FIG. 22A, light isemitted to the sealing substrate 2204 side. Therefore, in the case ofusing a light emitting element with a top emission structure to adisplay device, a substrate which transmits light is used as the sealingsubstrate 2204.

In the case of providing an optical film, an optical film may beprovided over the sealing substrate 2204.

In the case of the pixel structure shown in FIG. 11 of Embodiment Mode3, a metal film formed of a material which functions as a cathode andhas a low work function, such as MgAg, MgIn, or AlLi can be used for thefirst electrode 2402. For the second electrode 2404, alight-transmissive film such as an ITO (indium tin oxide) film or anindium zinc oxide (IZO) film can be used. Accordingly, with thisstructure, the transmittance of the top light emission can be improved.

Further, description is made with reference to FIG. 24B of a lightemitting element with a bottom emission structure. The same referencenumerals as those in FIG. 24A are used since the structures are the sameexcept for the light emission structure.

Here, as a material used for the first electrode 2402 which functions asan anode, a material having a high work function is preferably used. Forexample, a light-transmissive film such as an ITO (indium tin oxide)film or an indium zinc oxide (IZO) film can be used. By using alight-transmissive conductive film, an anode which can transmit lightcan be formed.

As a material used for the second electrode 2404 which functions as acathode, a metal film formed of a material having a low work function(Al, Ag, Li, Ca, or an alloy thereof such as MgAg, MgIn, AlLi, CaF₂, orCa₃N₂) can be used. By using a metal film which reflects light, acathode which does not transmit light can be formed.

In this manner, light from the light emitting element can be extractedto a bottom surface as shown by an arrow in FIG. 24B. That is, in thecase of applying to the display panel shown in FIGS. 22A and 22B, lightis emitted to the substrate 2210 side. Therefore, in the case of using alight emitting element with a bottom emission structure to a displaydevice, a substrate which transmits light is used as the substrate 2210.

In the case of providing an optical film, an optical film may beprovided over the substrate 2210.

Description is made with reference to FIG. 24C of a light emittingelement with a dual emission structure. The same reference numerals asthose in FIG. 24A are used since the structures are the same except forthe light emission structure.

Here, as a material used for the first electrode 2402 which functions asan anode, a material having a high work function is preferably used. Forexample, a light-transmissive film such as an ITO (indium tin oxide)film or an indium zinc oxide (IZO) film can be used. By using alight-transmissive conductive film, an anode which can transmit lightcan be formed.

As a material used for the second electrode 2404 which functions as acathode, a stacked layer of a metal thin film formed of a materialhaving a low work function (Al, Ag, Li, Ca, or an alloy thereof such asMgAg, MgIn, AlLi, CaF₂, or Ca₃N₂), and a light-transmissive conductivefilm (ITO (indium tin oxide), indium oxide zinc oxide (In₂O₃—ZnO) alloy,zinc oxide (ZnO), or the like) is preferably used. By using a metal thinfilm and a light-transmissive conductive film in this manner, a cathodewhich can transmit light can be formed.

In this manner, light from the light emitting element can be extractedto the both surfaces as shown by arrows of FIG. 24C. That is, in thecase of applying to the display panel shown in FIGS. 22A and 22B, lightis emitted to the substrate 2210 side and the sealing substrate 2204side. Therefore, in the case of applying a light emitting element with adual emission structure to a display device, a substrate which transmitslight is used as the substrate 2210 and the sealing substrate 2204.

In the case of providing an optical film, optical films may be providedover both the substrate 2210 and the sealing substrate 2204.

The invention can also be applied to a display device which realizesfull color display by using a white light emitting element and a colorfilter.

As shown in FIG. 25, a base film 2502 is formed over a substrate 2500and a driving TFT 2501 is formed thereover. A first electrode 2503 isformed in contact with a source electrode of the driving TFT 2501 and alayer 2504 containing an organic compound and a second electrode 2505are formed thereover.

The first electrode 2503 is an anode of a light emitting element. Thesecond electrode 2505 is a cathode of the light emitting element. Thatis, a region where the layer 2504 containing an organic compound isinterposed between the first electrode 2503 and the second electrode2505 corresponds to the light emitting element. In the structure shownin FIG. 25, white light is emitted. A red color filter 2506R, a greencolor filter 2506G and a blue color filter 2506B are provided over thelight emitting element, thereby full color display can be performed.Further, a black matrix (also referred to as BM) 2507 for separatingthese color filters is provided.

The aforementioned structures of the light emitting element can be usedin combination and can be appropriately used for the display devicehaving the pixel structure of the invention. The structures of thedisplay panel and the light emitting elements which are described aboveare examples and it is needless to say that the pixel structure of theinvention can be applied to display devices having other structures.

Next, a partial cross-sectional view of a pixel portion of a displaypanel is described.

First, description is made with reference to FIGS. 26A and 26B and FIGS.27A and 27B of the case of using a crystalline semiconductor film(polysilicon (p-Si:H) film) as a semiconductor layer of a transistor.

Here, the semiconductor layer is obtained by, for example, forming anamorphous silicon (a-Si) film over a substrate by a known filmdeposition method. Note that the semiconductor film is not limited tothe amorphous silicon film, and any semiconductor film having anamorphous structure (including a microcrystalline semiconductor film)may be used. Further, a compound semiconductor film having an amorphousstructure, such as an amorphous silicon germanium film may be used.

Then, the amorphous silicon film is crystallized by lasercrystallization, thermal crystallization using RTA or an annealingfurnace, thermal crystallization using a metal element which promotescrystallization, or the like. Needless to say, such crystallization maybe performed in combination.

As a result of the aforementioned crystallization, a crystallized regionis formed in a part of the amorphous semiconductor film.

In addition, the crystalline semiconductor film having a partiallyincreased crystallinity is patterned into a desired shape, and anisland-shaped semiconductor film (each of the films that are formed byseparating one semiconductor film) is formed with the crystallizedregion. This semiconductor film is used as the semiconductor layer ofthe transistor. Note that patterning is to process a film shape, whichmeans forming a film pattern by a photolithography technique (includingforming contact hole in photosensitive acrylic and processingphotosensitive acrylic so as to be a spacer), forming a mask pattern bya photolithography technique and etching with the use of the maskpattern, or the like.

As shown in FIG. 26A, a base film 26102 is formed over a substrate26101, and a semiconductor layer is formed thereover. The semiconductorlayer includes a channel forming region 26103 and an impurity region26105 functioning as a source or drain region, which are in a drivingtransistor 26118, and a channel forming region 26106, a lowconcentration impurity region 26107, and an impurity region 26108functioning as a lower electrode, which are in a capacitor 26119. Notethat channel doping may be performed to the channel forming regions26103 and 26106.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. The base film26102 can be formed using a single layer of aluminum nitride (AlN),silicon oxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like, orstacked layers thereof.

A gate electrode 26110 and an upper electrode 26111 of the capacitor areformed over the semiconductor layer with a gate insulating film 26109interposed therebetween.

An interlayer insulating film 26112 is formed so as to cover the drivingtransistor 26118 and the capacitor 26119. Then, a contact hole is formedin the interlayer insulating film 26112, through which a wire 26113 isin contact with the impurity region 26105. A pixel electrode 26114 isformed in contact with the wire 26113, and an interlayer insulator 26115is formed so as to cover end portions of the pixel electrode 26114 andthe wire 26113. Here, the interlayer insulator 26115 is formed with apositive photosensitive acrylic resin film. Then, a layer 26116containing an organic compound and an opposed electrode 26117 are formedover the pixel electrode 26114. Thus, a light emitting element 26120corresponds to a region where the layer 26116 containing an organiccompound is interposed between the pixel electrode 26114 and the opposedelectrode 26117.

In addition, as shown in FIG. 26B, a low concentration impurity region26202 may be provided so as to overlap the upper electrode 26111 in thelow concentration impurity region 26107 which form a part of the lowerelectrode of the capacitor 26119. That is, the lower electrode of thecapacitor 26119 is formed with a channel forming region 26201, the lowconcentration impurity regions 26202 and 26107, and an impurity region26108. Note that common portions to those in FIG. 26A are denoted by thesame reference numerals, and description thereof is omitted.

In addition, as shown in FIG. 27A, a second upper electrode 26301 may beprovided, which is formed in the same layer as the wire 26113 in contactwith the impurity region 26105 of the driving transistor 26118. Notethat common portions to those in FIG. 26A are denoted by the samereference numerals, and description thereof is omitted. A secondcapacitor is formed by interposing the interlayer insulating film 26112between the second upper electrode 26301 and the upper electrode 26111.In addition, since the second upper electrode 26301 is in contact withthe impurity region 26108, a first capacitor having such a structurethat the gate insulating film 26109 is interposed between the upperelectrode 26111 and the channel forming region 26106, and the secondcapacitor having such a structure that the interlayer insulating film26112 is interposed between the upper electrode 26111 and the secondupper electrode 26301 are connected in parallel, so that a capacitor26302 having the first and second capacitors is obtained. Since thecapacitor 26302 has a total capacitance of those of the first and secondcapacitors, the capacitor having a large capacitance can be formed in asmall area. That is, using the capacitor in the pixel structure of theinvention will lead to a further improved aperture ratio.

Alternatively, a structure of a capacitor as shown in FIG. 27B may beadopted. A base film 27102 is formed over a substrate 27101, and asemiconductor layer is formed thereover. The semiconductor layerincludes a channel forming region 27103 and an impurity region 27105 tofunction as a source or drain region of a driving transistor 27118. Notethat channel doping may be performed to the channel forming region27103.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. The base film27102 can be formed using a single layer of aluminum nitride (AlN),silicon oxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like orstacked layers thereof.

A gate electrode 27107 and a first electrode 27108 are formed over thesemiconductor layer with a gate insulating film 27106 interposedtherebetween.

A first interlayer insulating film 27109 is formed so as to cover thedriving transistor 27118 and the first electrode 27108. Then, a contacthole is formed in the first interlayer insulating film 27109, throughwhich a wire 27110 is in contact with the impurity region 27105. Inaddition, a second electrode 27111 is formed in the same layer and withthe same material as the wire 27110.

Furthermore, a second interlayer insulating film 27112 is formed so asto cover the wire 27110 and the second electrode 27111. Then, a contacthole is formed in the second interlayer insulating film 27112, throughwhich a pixel electrode 27113 is formed in contact with the wire 27110.A third electrode 27114 is formed in the same layer and with the samematerial as the pixel electrode 27113. Here, a capacitor 27119 is formedof the first electrode 27108, the second electrode 27111, and the thirdelectrode 27114.

An insulator 27115 is formed so as to cover an end portion of the pixelelectrode 27113 and the third electrode 27114, over which a layer 27116containing an organic compound and an opposed electrode 27117 areformed. Then, a light emitting element 27120 corresponds to a regionwhere the layer 27116 containing an organic compound is interposedbetween the pixel electrode 27113 and the opposed electrode 27117.

As described above, each of the structures shown in FIGS. 26A and 26Band FIGS. 27A and 27B can be given as a structure of a transistor usinga crystalline semiconductor film for its semiconductor layer. Note thatthe transistors having the structures shown in FIGS. 26A and 26B andFIGS. 27A and 27B are examples of transistors with a top-gate structure.That is, the transistor may be either a p-channel transistor or ann-channel transistor. In the case where the transistor is an n-channeltransistor, the LDD region may be formed either so as to overlap thegate electrode or not to overlap, or a part of the LDD region may beformed so as to overlap the gate electrode. Further, the gate electrodemay have a tapered shape and the LDD region may be provided below thetapered portion of the gate electrode in a self-aligned manner. Inaddition, the number of gate electrodes is not limited to two, and amulti-gate structure with three or more gate electrodes may be employed,or a single gate structure may also be employed.

By using a crystalline semiconductor film for a semiconductor layer(channel forming region, source region, drain region, or the like) of atransistor included in the pixel of the invention, for example, itbecomes easier to form the scan line driver circuit 402 and the signalline driver circuit 401 over the same substrate as the pixel portion 403in FIG. 4.

Next, as a structure of a transistor which uses polysilicon (p-Si) forits semiconductor layer, FIG. 30A shows a partial cross-sectional viewof a display panel using a transistor which has a structure where a gateelectrode is interposed between a substrate and a semiconductor layer,that is, a transistor with a bottom-gate structure where a gateelectrode is located below a semiconductor layer.

A base film 3002 is formed over a substrate 3001. Then, a gate electrode3003 is formed over the base film 3002. A first electrode 3004 is formedin the same layer and with the same material as the gate electrode. As amaterial of the gate electrode 3003, polycrystalline silicon to whichphosphorus is added can be used. Besides polycrystalline silicon,silicide which is a compound of metal and silicon may be used.

Then, a gate insulating film 3005 is formed so as to cover the gateelectrode 3003 and the first electrode 3004. As the gate insulating film3005, a silicon oxide film, a silicon nitride film, or the like is used.

A semiconductor layer is formed over the gate insulating film 3005. Thesemiconductor layer includes a channel forming region 3006, an LDDregion 3007, and an impurity region 3008 functioning as a source ordrain region, which are in a driving transistor 3022, and a channelforming region 3009, an LDD region 3010, and an impurity region 3011,which function as a second electrode of a capacitor 3023. Note thatchannel doping may be performed to the channel forming regions 3006 and3009.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. The base film3002 can be formed using a single layer of aluminum nitride (AlN),silicon oxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like orstacked layers thereof.

A first interlayer insulating film 3012 is formed so as to cover thesemiconductor layer. Then, a contact hole is formed in the firstinterlayer insulating film 3012, through which a wire 3013 is in contactwith the impurity region 3008. A third electrode 3014 is formed in thesame layer and with the same material as the wire 3013. The capacitor3023 is formed with the first electrode 3004, the second electrode, andthe third electrode 3014.

In addition, an opening portion 3015 is formed in the first interlayerinsulating film 3012. A second interlayer insulating film 3016 is formedso as to cover the driving transistor 3022, the capacitor 3023, and theopening portion 3015. Then, a contact hole is formed in the secondinterlayer insulating film 3016, through which a pixel electrode 3017 isformed. Then, an insulator 3018 is formed so as to cover end portions ofthe pixel electrode 3017. For example, a positive photosensitive acrylicresin film can be used. Subsequently, a layer 3019 containing an organiccompound and an opposed electrode 3020 are formed over the pixelelectrode 3017. Thus, a light emitting element 3021 corresponds to aregion where the layer 3019 containing an organic compound is interposedbetween the pixel electrode 3017 and the opposed electrode 3020. Theopening portion 3015 is located below the light emitting element 3021.That is, in the case where light emitted from the light emitting element3021 is extracted from the substrate side, the transmittance can beimproved due to the existence of the opening portion 3015.

Furthermore, a fourth electrode 3024 may be formed in the same layer andwith the same material as the pixel electrode 3017 in FIG. 30A so as toobtain a structure shown in FIG. 30B. In this case, a capacitor 3025 canbe formed with the first electrode 3004, the second electrode, the thirdelectrode 3014, and the fourth electrode 3024.

Next, description is made of the case of using an amorphous silicon(a-Si:H) film as a semiconductor layer of a transistor. FIGS. 31A and31B show cases of a top-gate transistor, and FIGS. 32A, 32B, 30A, and30B show cases of a bottom-gate transistor.

FIG. 31A shows a cross sectional view of a transistor having a forwardstaggered structure, which uses amorphous silicon for its semiconductorlayer. A base film 3102 is formed over a substrate 3101. Further, apixel electrode 3103 is formed over the base film 3102. In addition, afirst electrode 3104 is formed in the same layer and with the samematerial as the pixel electrode 3103.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. The base film3102 can be formed using a single layer of aluminum nitride (AlN),silicon oxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like orstacked layers thereof.

Wires 3105 and 3106 are formed over the base film 3102, and an endportion of the pixel electrode 3103 is covered with the wire 3105.N-type semiconductor layers 3107 and 3108 each having N-typeconductivity are formed over the wires 3105 and 3106 respectively. Inaddition, a semiconductor layer 3109 is formed between the wires 3105and 3106 and over the base film 3102, which is partially extended so asto cover the N-type semiconductor layers 3107 and 3108. Note that thissemiconductor layer is formed with an amorphous semiconductor film suchas an amorphous silicon (a-Si:H) film or a microcrystallinesemiconductor (μ-Si:H) film. Then, a gate insulating film 3110 is formedover the semiconductor layer 3109, and an insulating film 3111 is formedin the same layer and with the same material as the gate insulating film3110, and also over the first electrode 3104. Note that as the gateinsulating film 3110, a silicon oxide film, a silicon nitride film, orthe like is used.

A gate electrode 3112 is formed over the gate insulating film 3110. Inaddition, a second electrode 3113 is formed in the same layer and withthe same material as the gate electrode, and over the first electrode3104 with the insulating film 3111 interposed therebetween. A capacitor3119 corresponds to a region where the insulating film 3111 isinterposed between the first electrode 3104 and the second electrode3113. An insulator 3114 is formed so as to cover end portions of thepixel electrode 3103, the driving transistor 3118, and the capacitor3119.

A layer 3115 containing an organic compound and an opposed electrode3116 are formed over the insulator 3114 and the pixel electrode 3103located in an opening portion of the insulator 3114. Thus, a lightemitting element 3117 corresponds to a region where the layer 3115containing an organic compound is interposed between the pixel electrode3103 and the opposed electrode 3116.

The first electrode 3104 shown in FIG. 31A may be formed like a firstelectrode 3120 as shown in FIG. 31B. The first electrode 3120 is formedin the same layer and with the same material as the wires 3105 and 3106.

FIGS. 32A and 32B are partial cross-sectional views of a display panelhaving a bottom-gate transistor which uses amorphous silicon as itssemiconductor layer.

A base film 3202 is formed over a substrate 3201. A gate electrode 3203and a first electrode 3204 formed in the same layer and with the samematerial over the base film 3202. As a material of the gate electrode3203, polycrystalline silicon to which phosphorus is added can be used.Besides polycrystalline silicon, silicide which is a compound of metaland silicon may be used.

Then, a gate insulating film 3205 is formed so as to cover the gateelectrode 3203 and the first electrode 3204. As the gate insulating film3205, a silicon oxide film, a silicon nitride film, or the like is used.

A semiconductor layer 3206 is formed over the gate insulating film 3205.In addition, a semiconductor layer 3207 is formed in the same layer andwith the same material as the semiconductor layer 3206.

As the substrate, a glass substrate, a quartz substrate, a ceramicsubstrate, a plastic substrate, or the like can be used. The base film3202 can be formed using a single layer of aluminum nitride (AlN),silicon oxide (SiO₂), silicon oxynitride (SiO_(x)N_(y)), or the like orstacked layers thereof.

N-type semiconductor layers 3208 and 3209 having N-type conductivity areformed over the semiconductor layer 3206, and an N-type semiconductorlayer 3210 is formed over the semiconductor layer 3207.

Wires 3211 and 3212 are formed over the N-type semiconductor layers 3208and 3209 respectively, and a conductive layer 3213 is formed in the samelayer and with the same material as the wires 3211 and 3212, over theN-type semiconductor layer 3210.

Thus, a second electrode is formed with the semiconductor layer 3207,the N-type semiconductor layer 3210, and the conductive layer 3213. Notethat a capacitor 3220 having a structure where the gate insulating film3205 is interposed between the second electrode and the first electrode3204 is formed.

One end portion of the wire 3211 is extended, and a pixel electrode 3214is formed so as to be in contact with an upper portion of the extendedwire 3211.

In addition, an insulator 3215 is formed so as to cover end portions ofthe pixel electrode 3214, a driving transistor 3219, and the capacitor3220.

Then, a layer 3216 containing an organic compound and an opposedelectrode 3217 are formed over the pixel electrode 3214 and theinsulator 3215. A light emitting element 3218 corresponds to a regionwhere the layer 3216 containing an organic compound is interposedbetween the pixel electrode 3214 and the opposed electrode 3217.

The semiconductor layer 3207 and the N-type semiconductor layer 3210 tobe a part of the second electrode of the capacitor are not necessarilyrequired. That is, the second electrode may be the conductive layer3213, so that the capacitor may have such a structure that the gateinsulating film is interposed between the first electrode 3204 and theconductive layer 3213.

Note that the pixel electrode 3214 is formed before forming the wire3211 in FIG. 32A, thereby a capacitor 3222 as shown in FIG. 32B can beobtained, which has a structure where the gate insulating film 3205 isinterposed between the first electrode 3204 and a second electrode 3221formed of the pixel electrode 3214.

Although FIGS. 32A and 32B show inverted staggered channel-etchedtransistors, a channel-protective transistor may be used. Description ofchannel-protective transistors is made with reference to FIGS. 33A and33B.

A channel-protective transistor shown in FIG. 33A is different from thechannel-etched driving transistor 3219 shown in FIG. 32A in that aninsulator 3301 functioning as an etching mask is provided over thechannel forming region in the semiconductor layer 3206. Common portionsexcept that point are denoted by the same reference numerals.

Similarly, a channel-protective transistor shown in FIG. 33B isdifferent from the channel-etched driving transistor 3219 shown in FIG.32B in that the insulator 3301 functioning as an etching mask isprovided over the channel forming region in the semiconductor layer3206. Common portions except that point are denoted by the samereference numerals.

By using an amorphous semiconductor film as a semiconductor layer(channel forming region, source region, drain region, and the like) of atransistor included in the pixel of the invention, the manufacturingcost can be reduced. For example, an amorphous semiconductor film can beapplied by using the pixel structure shown in FIG. 2,

Note that structures of the transistors and capacitors to which thepixel structure of the invention can be applied are not limited to thosedescribed above, and various structures of transistors and capacitorscan be used.

(Embodiment Mode 7)

The display device of the invention can be applied to various electronicappliances, specifically a display portion of electronic appliances. Theelectronic appliances include cameras such as a video camera and adigital camera, a goggle-type display, a navigation system, an audioreproducing device (car audio component stereo, audio component stereo,or the like), a computer, a game machine, a portable informationterminal (mobile computer, mobile phone, mobile game machine, electronicbook, or the like), an image reproducing device having a recordingmedium (specifically, a device for reproducing a recording medium suchas a digital versatile disc (DVD) and having a display for displayingthe reproduced image) and the like.

FIG. 34A shows a display which includes a housing 34001, a supportingbase 34002, a display portion 34003, a speaker portion 34004, a videoinputting terminal 34005, and the like. A display device having thepixel structure of the invention can be used for the display portion34003. Note that the display includes all display devices for displayinginformation such as for a personal computer, receiving televisionbroadcasting, and displaying an advertisement. A display using thedisplay device having the pixel structure of the invention for thedisplay portion 34003 can reduce power consumption and prevent a displaydefect. Further, cost reduction can be achieved.

In recent years, the need to grow in size of a display has beenincreased. In accordance with the enlargement of a display, rise inprice becomes a problem. Therefore, it is an object to reduce themanufacturing cost as much as possible and a high quality product isprovided at as low price as possible.

For example, by applying the pixel structure of FIG. 2, FIG. 11, or thelike to a pixel portion of a display panel, a display panel formed withunipolar transistors can be provided. Therefore, manufacturing steps canbe reduced, which leads to reduction in the manufacturing cost.

In addition, by forming the pixel portion and the peripheral drivercircuit over the same substrate as shown in FIG. 22A, the display panelcan be constituted by circuits including unipolar transistors.

In addition, by using an amorphous semiconductor (such as amorphoussilicon (a-Si:H)) as a semiconductor layer of a transistor in a circuitconstituting the pixel portion, manufacturing steps can be simplifiedand further cost reduction can be realized. In this case, it ispreferable that a driver circuit in the periphery of the pixel portionbe formed into an IC chip and mounted on the display panel by COG or thelike as shown in FIGS. 28B and 29A. In this manner, by using anamorphous semiconductor, it becomes easy to grow in size of the display.

FIG. 34B shows a camera which includes a main body 34101, a displayportion 34102, an image receiving portion 34103, operating keys 34104,an external connection port 34105, a shutter 34106, and the like.

In recent years, in accordance with advance in performance of a digitalcamera and the like, competitive manufacturing thereof has beenintensified. Thus, it is important to provide a higher-performanceproduct at as low price as possible. A digital camera using a displaydevice having a pixel structure of the invention for the display portion34102 can reduce power consumption and prevent a display defect.Further, cost reduction can be achieved.

By using the pixel structure of FIG. 2 or FIG. 11 for the pixel portion,the pixel portion can be constituted by unipolar transistors. Inaddition, as shown in FIG. 28A, by forming a signal line driver circuitwhose operating speed is high into an IC chip, and forming a scan linedriver circuit whose operating speed is relatively low with a circuitconstituted by unipolar transistors over the same substrate as the pixelportion, higher performance can be realized and cost reduction can beachieved. In addition, by using an amorphous semiconductor such asamorphous silicon for a semiconductor layer of a transistor in the pixelportion and the scan line driver circuit formed over the same substrateas the pixel portion, further cost reduction can be achieved.

FIG. 34C shows a computer which includes a main body 34201, a housing34202, a display portion 34203, a keyboard 34204, an external connectionport 34205, a pointing mouse 34206, and the like. A computer using adisplay device having the pixel structure of the invention for thedisplay portion 34203 can reduce power consumption and prevent a displaydefect. Further, cost reduction can be achieved.

FIG. 34D shows a mobile computer which includes a main body 34301, adisplay portion 34302, a switch 34303, operating keys 34304, an infraredport 34305, and the like. A mobile computer using a display devicehaving a pixel structure of the invention for the display portion 34302can reduce power consumption and prevent a display defect. Further, costreduction can be achieved.

FIG. 34E shows a portable image reproducing device having a recordingmedium (specifically, a DVD reproducing device), which includes a mainbody 34401, a housing 34402, a display portion A 34403, a displayportion B 34404, a recording medium (DVD or the like) reading portion34405, an operating key 34406, a speaker portion 34407, and the like.The display portion A 34403 mainly displays image data and the displayportion B 34404 mainly displays text data. An image reproducing deviceusing a display device having a pixel structure of the invention for thedisplay portions A 34403 and B 34404 can reduce power consumption andprevent a display defect. Further, cost reduction can be achieved.

FIG. 34F shows a goggle-type display which includes a main body 34501, adisplay portion 34502, and an arm portion 34503. A goggle type displayusing a display device having a pixel structure of the invention for thedisplay portion 34502 can reduce power consumption and prevent a displaydefect. Further, cost reduction can be achieved.

FIG. 34G shows a video camera which includes a main body 34601, adisplay portion 34602, a housing 34603, an external connection port34604, a remote control receiving portion 34605, an image receivingportion 34606, a battery 34607, an audio input portion 34608, operatingkeys 34609, an eye piece portion 34610, and the like. A video camerausing a display device having a pixel structure of the invention for thedisplay portion 34602 can reduce power consumption and prevent a displaydefect. Further, cost reduction can be achieved.

FIG. 34H shows a mobile phone which includes a main body 34701, ahousing 34702, a display portion 34703, an audio inputting portion34704, an audio outputting portion 34705, operating keys 34706, anexternal connection port 34707, an antenna 34708, and the like.

In recent years, a mobile phone has been provided with a game function,a camera function, an electronic money function, or the like, and theneed of a high-value added mobile phone has been increased. Further, ahigh-definition display has been demanded. A mobile phone using adisplay device having the pixel structure of the invention for thedisplay portion 34703 can reduce power consumption and prevent a displaydefect. Further, an aperture ratio of a pixel is increased, and highdefinition display can be performed. In addition, cost reduction can beachieved.

For example, by applying the pixel structure of FIG. 2 for the pixelportion, an aperture ratio of a pixel can be improved. Specifically, byusing an n-channel transistor as a driving transistor for driving alight emitting element, the aperture ratio of the pixel is improved.Consequently, a mobile phone having a high-definition display portioncan be provided.

In addition, since the aperture ratio is improved, a high-value addedmobile phone having a high-definition display portion can be provided byusing the display device having a dual emission structure as shown inFIG. 24C for the display portion.

While a mobile phone has been multifunctional and frequency of usethereof has been increased, the life per charge has been required to belong.

For example, by forming a peripheral driver circuit into an IC chip asshown in FIG. 28B and FIG. 29A and using a CMOS or the like, powerconsumption can be reduced.

Thus, the invention can be applied to various electronic appliances.

(Embodiment Mode 8)

In this embodiment mode, description is made with reference to FIG. 37of an example of a structure of a mobile phone which has a displayportion having a display device using a pixel structure of theinvention.

A display panel 3710 is incorporated in a housing 3700 so as to befreely attached and detached. The shape and size of the housing 3700 canbe appropriately changed in accordance with the size of the displaypanel 3710. The housing 3700 provided with the display panel 3710 isfitted in a printed circuit board 3701 so as to be assembled as amodule.

The display panel 3710 is connected to the printed circuit board 3701through an FPC 3711. A speaker 3702, a microphone 3703, a transmittingand receiving circuit 3704, and a signal processing circuit 3705including a CPU, a controller, and the like are formed over the printedcircuit board 3701. Such a module, an inputting means 3706, and abattery 3707 are combined, which is stored in a housing 3709. A pixelportion of the display panel 3710 is disposed so as to be seen from anopening window formed in the housing 3709.

The display panel 3710 may be formed by forming a pixel portion and apart of peripheral driver circuits (a driver circuit whose operationfrequency is low among a plurality of driver circuits) using TFTs overthe same substrate; forming a part of the peripheral driver circuits (adriver circuit whose operation frequency is high among the plurality ofdriver circuits) into an IC chip; and mounting the IC chip on thedisplay panel 3710 by COG (Chip On Glass). The IC chip may be,alternatively, connected to a glass substrate by using TAB (TapeAutomated Bonding) or a printed circuit board. It is to be noted thatFIG. 28A shows an example of structure of such a display panel that apart of peripheral driver circuits is formed over the same substrate asa pixel portion and an IC chip provided with the other part of theperipheral driver circuits is mounted by COG or the like. By employingthe above-described structure, power consumption of a display device canbe reduced and the life per charge of a mobile phone can be made long.In addition, cost reduction of the mobile phone can be achieved.

To the pixel portion, the pixel structures described in Embodiment Modes1 to 4 can be appropriately applied.

For example, by applying the pixel structure of FIG. 2 described inEmbodiment Mode 1 or the pixel structure of FIG. 11 described inEmbodiment Mode 3, the manufacturing steps can be reduced. That is tosay, the pixel portion and the peripheral driver circuit formed over thesame substrate as the pixel portion are constituted by unipolartransistors in order to achieve cost reduction.

In addition, in order to further reduce the power consumption, the pixelportion may be formed using TFT's over a substrate, all of theperipheral driver circuits may be formed into an IC chip, and the ICchip may be mounted on the display panel by COG (Chip On Glass) or thelike as shown in FIGS. 28B and 29A. The pixel structure of FIG. 2 isused for the pixel portion and an amorphous semiconductor film is usedfor a semiconductor layer of a transistor, thereby reducingmanufacturing cost.

It is to be noted that the structure described in this embodiment modeis an example of a mobile phone, and the pixel structure of theinvention can be applied not only to a mobile phone having theabove-described constitution but also to mobile phones having variousstructures.

(Embodiment Mode 9)

FIG. 35 shows an EL module combining a display panel 3501 and a circuitboard 3502. The display panel 3501 includes a pixel portion 3503, a scanline driver circuit 3504, and a signal line driver circuit 3505. Acontrol circuit 3506, a signal dividing circuit 3507, and the like areformed over the circuit board 3502. The display panel 3501 and thecircuit board 3502 are connected to each other by a connecting wire3508. As the connecting wire, an FPC or the like can be used.

The display panel 3501 may be formed by forming a pixel portion and apart of peripheral driver circuits (a driver circuit whose operationfrequency is low among a plurality of driver circuits) using TFTs overthe same substrate; forming a part of the peripheral driver circuits (adriver circuit whose operation frequency is high among the plurality ofdriver circuits) into an IC chip; and mounting the IC chip on thedisplay panel 3501 by COG (Chip On Glass) or the like. The IC chip maybe, alternatively, mounted on the display panel 3501 by using TAB (TapeAutomated Bonding) or a printed circuit board. It is to be noted thatFIG. 28A shows an example of structure where a part of peripheral drivercircuits is formed over the same substrate as a pixel portion and an ICchip provided with the other part of the peripheral driver circuits ismounted by COG or the like.

In the pixel portion, the pixel structures described in Embodiment Modes1 to 4 can be appropriately applied.

For example, by applying the pixel structure of FIG. 2 described inEmbodiment Mode 1 or the pixel structure of FIG. 11 described inEmbodiment Mode 3, the manufacturing steps can be reduced. That is tosay, the pixel portion and the peripheral driver circuit formed over thesame substrate as the pixel portion are constituted by unipolartransistors in order to achieve cost reduction.

In addition, in order to further reduce the power consumption, the pixelportion may be formed using TFTs over a glass substrate, all of theperipheral driver circuits may be formed into an IC chip, and the ICchip may be mounted on the display panel by COG (Chip On Glass) or thelike.

In addition, by applying the pixel structure shown in FIG. 2 ofEmbodiment Mode 1, pixels can be constituted only by n-channeltransistors, so that an amorphous semiconductor (such as amorphoussilicon) can be applied to a semiconductor layer of a transistor. Thatis, a large display device where it is difficult to form an evencrystalline semiconductor film can be manufactured. Further, by using anamorphous semiconductor film for a semiconductor layer of a transistorconstituting a pixel, the manufacturing steps can be reduced andreduction in the manufacturing cost can be achieved.

It is preferable that, in the case where an amorphous semiconductor filmis applied to a semiconductor layer of a transistor constituting apixel, the pixel portion be formed using TFTs over a substrate, all ofthe peripheral driver circuits be formed into an IC chip, and the ICchip be mounted on the display panel by COG (Chip On Glass). Note thatFIG. 28B shows an example of the structure where a pixel portion isformed over a substrate and an IC chip provided with a peripheral drivercircuit is mounted on the substrate by COG or the like.

An EL television receiver can be completed with the above-described ELmodule. FIG. 36 is a block diagram showing a main structure of an ELtelevision receiver. A tuner 3601 receives a video signal and an audiosignal. The video signals are processed by a video signal amplifiercircuit 3602, a video signal processing circuit 3603 for converting asignal outputted from the video signal amplifier circuit 3602 into acolor signal corresponding to each color of red, green and blue, and thecontrol circuit 3506 for converting the video signal into the inputspecification of a driver circuit. The control circuit 3506 outputs asignal to each of the scan line side and the signal line side. In thecase of driving in a digital manner, a structure where the signaldividing circuit 3507 is provided on the signal line side to supply aninput digital signal by dividing into m signals may be employed.

An audio signal received by the tuner 3601 is transmitted to an audiosignal amplifier circuit 3604, an output thereof is supplied to aspeaker 3606 through an audio signal processing circuit 3605. A controlcircuit 3607 receives receiving station (received frequency) and volumecontrol data from an input portion 3608, and transmits signals to thetuner 3601 and the audio signal processing circuit 3605.

By incorporating the EL module shown in FIG. 35 into the housing 34001,a TV receiver can be completed as shown in FIG. 34A. The display portion34003 is constituted by the EL module. In addition, the speaker portion34004, the video inputting terminal 34005, and the like are providedappropriately.

It is needless to say that the invention can also be applied to variousappliances other than the TV receiver, such as a monitor of a personalcomputer, and in particular a large display medium such as aninformation display panel at the station or the airport, and anadvertisement board on the street.

This application is based on Japanese Patent Application serial no.2005-191145 filed in Japan Patent Office on 30 Jun. 2005, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a pixel comprising:a light emitting element; a first transistor; a second transistor; athird transistor; and a capacitor, wherein one of a source and a drainof the first transistor is directly connected to the light emittingelement or is indirectly connected to the light emitting element throughone or more electrical elements other than the first transistor, whereina gate of the first transistor is directly connected to a firstelectrode of the capacitor or is indirectly connected to the firstelectrode of the capacitor through one or more electrical elements otherthan the first transistor or the capacitor, wherein a second electrodeof the capacitor is directly connected to the light emitting element oris indirectly connected to the light emitting element through one ormore electrical elements other than the capacitor, wherein one of asource and a drain of the second transistor is directly connected to thelight emitting element or is indirectly connected to the light emittingelement through one or more electrical elements other than the secondtransistor, wherein one of a source and a drain of the third transistoris directly connected to the gate of the first transistor or isindirectly connected to the gate of the first transistor through one ormore electrical elements other than the third transistor or the firsttransistor, wherein the other of the source and the drain of the firsttransistor is directly connected to a first wire or is indirectlyconnected to the first wire through one or more electrical elementsother than the first transistor, wherein the other of the source and thedrain of the second transistor is directly connected to a second wire oris indirectly connected to the second wire through one or moreelectrical elements other than the second transistor, wherein the otherof the source and the drain of the third transistor is directlyconnected to a third wire or is indirectly connected to the third wirethrough one or more electrical elements other than the third transistor,wherein each of the first transistor, the second transistor and thethird transistor comprises In, Ga, Zn and O, wherein the light emittingelement is configured to emit white light, and wherein a color filterand the light emitting element overlap each other.
 2. The display deviceaccording to claim 1, further comprising a fourth transistor directlyconnected to the first transistor in parallel.
 3. The display deviceaccording to claim 1, wherein the first wire is constantly supplied witha predetermined potential.
 4. The display device according to claim 1,wherein the second wire is directly connected to a current source or isindirectly connected to the current source through one or moreelectrical elements.
 5. The display device according to claim 4, whereinthe currnt source is configured to set a signal current.
 6. The displaydevice according to claim 1, wherein the third wire is constantlysupplied with a predetermined potential.
 7. A display device comprising:a pixel comprising: a light emitting element; a first transistor; asecond transistor; a third transistor; and a capacitor, wherein one of asource and a drain of the first transistor is directly connected to thelight emitting element or is indirectly connected to the light emittingelement through one or more electrical elements other than the firsttransistor, wherein a gate of the first transistor is directly connectedto a first electrode of the capacitor or is indirectly connected to thefirst electrode of the capacitor through one or more electrical elementsother than the first transistor or the capacitor, wherein a secondelectrode of the capacitor is directly connected to the light emittingelement or is indirectly connected to the light emitting element throughone or more electrical elements other than the capacitor, wherein one ofa source and a drain of the second transistor is directly connected tothe light emitting element or is indirectly connected to the lightemitting element through one or more electrical elements other than thesecond transistor, wherein one of a source and a drain of the thirdtransistor is directly connected to the gate of the first transistor oris indirectly connected to the gate of the first transistor through oneor more electrical elements other than the third transistor or the firsttransistor, wherein the other of the source and the drain of the firsttransistor is directly connected to a first wire or is indirectlyconnected to the first wire through one or more electrical elementsother than the first transistor, wherein the other of the source and thedrain of the second transistor is directly connected to a second wire oris indirectly connected to the second wire through one or moreelectrical elements other than the second transistor, wherein the otherof the source and the drain of the third transistor is directlyconnected to a third wire or is indirectly connected to the third wirethrough one or more electrical elements other than the third transistor,wherein each of the first transistor, the second transistor and thethird transistor comprises In, Ga, Zn and O, wherein each of the firsttransistor, the second transistor and the third transistor comprises afirst gate electrode, a second gate electrode and a channel regionbetween the first gate electrode and the second gate electrode, whereinthe light emitting element is configured to emit white light, andwherein a color filter and the light emitting element overlap eachother.
 8. The display device according to claim 7, further comprising afourth transistor directly connected to the first transistor inparallel.
 9. The display device according to claim 7, wherein the firstwire is constantly supplied with a predetermined potential.
 10. Thedisplay device according to claim 7, wherein the second wire is directlyconnected to a current source or is indirectly connected to the currentsource through one or more electrical elements.
 11. The display deviceaccording to claim 10, wherein the currnt source is configured to set asignal current.
 12. The display device according to claim 7, wherein thethird wire is constantly supplied with a predetermined potential.
 13. Adisplay device comprising: a pixel comprising: a light emitting element;a first transistor; a second transistor; a third transistor; and acapacitor, wherein one of a source and a drain of the first transistoris directly connected to the light emitting element or is indirectlyconnected to the light emitting element through one or more electricalelements other than the first transistor, wherein a gate of the firsttransistor is directly connected to a first electrode of the capacitoror is indirectly connected to the first electrode of the capacitorthrough one or more electrical elements other than the first transistoror the capacitor, wherein a second electrode of the capacitor isdirectly connected to the light emitting element or is indirectlyconnected to the light emitting element through one or more electricalelements other than the capacitor, wherein one of a source and a drainof the second transistor is directly connected to the light emittingelement or is indirectly connected to the light emitting element throughone or more electrical elements other than the second transistor,wherein one of a source and a drain of the third transistor is directlyconnected to the gate of the first transistor or is indirectly connectedto the gate of the first transistor through one or more electricalelements other than the third transistor or the first transistor,wherein the other of the source and the drain of the first transistor isdirectly connected to a first wire or is indirectly connected to thefirst wire through one or more electrical elements other than the firsttransistor, wherein the other of the source and the drain of the secondtransistor is directly connected to a second wire or is indirectlyconnected to the second wire through one or more electrical elementsother than the second transistor, wherein the other of the source andthe drain of the third transistor is directly connected to a third wireor is indirectly connected to the third wire through one or moreelectrical elements other than the third transistor, wherein each of thefirst transistor, the second transistor and the third transistorcomprises In, Ga, Zn and O, wherein the light emitting element isconfigured to emit white light, wherein a color filter and the lightemitting element overlap each other, wherein the first transistorcomprises a gate electrode, a gate insulating film over the gateelectrode and a semiconductor layer over the gate insulating film,wherein an insulating film is over the semiconductor layer, wherein theinsulating film comprises a contact hole, wherein the first wire is overthe insulating film, wherein the first wire is in contact with thesemiconductor layer through the contact hole, and wherein the insulatingfilm is in direct contact with the semiconductor layer and the gateinsulating film.
 14. The display device according to claim 13, furthercomprising a fourth transistor directly connected to the firsttransistor in parallel.
 15. The display device according to claim 13,wherein the first wire is constantly supplied with a predeterminedpotential.
 16. The display device according to claim 13, wherein thesecond wire is directly connected to a current source or is indirectlyconnected to the current source through one or more electrical elements.17. The display device according to claim 16, wherein the currnt sourceis configured to set a signal current.
 18. The display device accordingto claim 13, wherein the third wire is constantly supplied with apredetermined potential.
 19. The display device according to claim 1,wherein the color filter is over the light emitting element.
 20. Thedisplay device according to claim 7, wherein the color filter is overthe light emitting element.
 21. The display device according to claim13, wherein the color filter is over the light emitting element.